Abstract: Abstract: Irrigation uniformity is the core evaluation index of drip irrigation. The influence factors of irrigation uniformity include pressure along mainstream way, emitter manufacturing deviation and emitter clogging, etc., but the leading factor is the longitudinal pressure distribution. Therefore, calculation of pressure is prerequisite for hydraulic design of drip irrigation system, and it is also the most basic problem for the study of drip irrigation hydraulics. Scholars at home and abroad have done a lot of research on this problem. According to the author's references, all the past research results were based on the energy conservation law, researchers mainly calculated the frictional head loss to determine the pressure distribution along drip irrigation pipe, and the friction resistance was the default for the only factor affecting the pressure distribution. However, this method neglects the energy of the fluid flowing through sidewards orifices, and it is not completely applicable to the drip irrigation pipe. To reveal the behavior of variable mass flow, and simplify the hydraulic calculation of drip irrigation pipe, this study established the basic equation of variable mass flow based on mass conservation and momentum conservation theories, and thus developed an analytical model of pressure distribution along mainstream in drip irrigation pipe. Momentum equation of variable mass flow is clearer in physical meaning, and it does not require to investigate the complex flow mechanism in detail, but rather rationally simplify uncertain factors. The developed model shows longitudinal pressure profile in drip irrigation pipe is determined by the friction loss and the momentum exchange, the friction loss tends to decrease the pressure while the momentum exchange just the opposite. The solution of pressure in mainstream way is attributed to determination of friction coefficient and momentum exchange factor. The friction coefficient of drip irrigation pipes could be determined by Blasius resistance formula, according to many existing research conclusions. Function form of momentum exchange factor was qualitatively analyzed, and it was represented by area contraction ratio caused by in-line emitters and relative axial location. A series of indoor pressure tests were performed on 6 types of drip irrigation pipes with different lengths, different emitter spaces and different operating pressures. According to the test result, axial velocity distribution index was obtained by regression, and variance analysis of 2 factors was performed, 2 possible influencing factors were emitter type and number of emitters. Variance analysis results showed that at a significance level of 5%, emitter types did not have a significant impact on axial velocity distribution index, meaning that with the condition of same number of emitters, axial velocity distribution indexes of different emitter types had no significant statistical differences. However, test results showed axial velocity distribution index was linearly dependent on number of emitters. Empirical expression of momentum exchange factor was regressed. Combined with the Blasius formula, the momentum equation of variable mass flow was solved, and pressure distribution along main stream in drip irrigation pipe was obtained. Calculated values of longitudinal pressure agreed well with measured values among all cases, and the maximum relative error was 4.27%. Although drip irrigation has been widely applied, research of flow characteristics in drip irrigation pipe is still inadequate, corresponding hydraulic calculations tend to mechanically adopt old methods, regardless of the applicability. This study offers a idea for hydraulic calculation of drip irrigation, the results will provide scientific evidence not only for structural and operational optimization of drip irrigation, but also for hydrodynamics study of multiple outlet pipes. What needs to be improved is the obtained pressure distribution model is not simple enough, so further research should perfect the flow mechanism, and measure relevant parameters more widely to simplify the current model.