Abstract: Abstract: Soil water content is an important surface variable for describing the soil dryness. Soil water content estimation is used in the monitoring of extreme hydrologic events, weather prediction, irrigation management and water balance modeling. The phenomena of soil moisture expansion, drying shrinkage and bulk density change are obvious in the land-surface hydrological processes. In terms of long-term soil water content monitoring, if only considering the change of mass water content and ignoring the volume-weight change, it will affect the measurement accuracy of soil volumetric water content and information estimation of water movement. Improving the measurement accuracy of soil volumetric water content is conducive to the optimization of irrigation planning and other agricultural practice managements. Ultrasonic pulse velocity sensing can provide a way for nondestructive sensing, which is widely used in the fields of biomedical research, concrete cement concentration detection, and engineering quality assessment. At present, the quantitative effects of soil bulk density on soil water content estimation under different conditions have not been reported in the measurement of soil water content using ultrasonic pulse velocity. The statistical quantitative analysis with the samples of red soil, lateritic red soil and paddy soil from Guangdong Province was carried out in this study. An ultrasonic soil water content detector was developed by using the ZBL-U510 type nonmetal ultrasonic detector. Considering the importance of parametric variation in soil water content in a range of temperature field, under 3 different ambient temperature conditions (10, 20 and 30 ℃), the effect of soil water content content on ultrasonic velocity was obtained and a mathematical model was established, which represented the relationship between soil water content and differential ultrasonic velocity considering the temperature effect. The equations representing the relationship between ultrasonic velocity and soil volumetric water content were presented. It could be found that it was nonlinear relationship with very high correlation. The regression analysis showed that the coefficients of determination for 3 soil types were greater than 0.973, and the RMS (root mean square) errors were small. It indicated that Richards model is feasible to be applied to describe the relationship between soil water content and ultrasonic speed value. The experimental results illustrated that the gradient of volume will be changed from gentle to sharp due to soil absorption. Compared to the cases of red soil and lateritic red soil, the variation range of bulk density of paddy soil was the widest. The reason was that paddy soil had higher clay content (>30%) and more expansive clay minerals than red soil and lateritic red soil, which resulted in an enhancement in the expansion of soil. The ultrasonic velocity in the case of paddy soil was faster than the cases of red soil and lateritic red soil. It was evident that the ultrasonic velocity was affected by soil composition and mechanical behavior to a certain extent. The results showed that the higher the soil viscosity and clay content, the faster the ultrasonic wave propagated. Also, the effect of temperature on ultrasonic velocity for different soil water contents was significant. The propagation speed of ultrasonic wave in the soil was the fastest under the 20 ℃ temperature condition, the second was under the 10 ℃ condition, and the slowest was under the 30 ℃ condition. The Richards model was proposed to characterize the relationship between soil volumetric water content and differential ultrasonic velocity, which showed a high correlation and its prediction error was around 3%. The capability of the proposed segmented temperature effect model was proved and its prediction error was less than 5%. This study confirms that the soil water content estimation model based on ultrasonic pulse velocity considering the temperature effect can be used to predict the water content of the soil under variable thermal conditions. This study can provide a reference for the application of ultrasonic technology in the field of soil water content detection.