Abstract: Abstract: Rapeseed and wheat are important oil and food crops in china, the sowing dates of them are adjacent, and planting area ranks in the forefront of the world. Precision combined seeding can improve working efficiency, reduce operating costs, increase farmer's income. Monitoring the sowing process is one of the trends in the development of intelligent seeders. The sowing process of the precision planter for rapeseed and wheat is completely closed, miss seeding caused by various factors in the field can not be corrected in time. It is of great practical significance to study a seed flow monitoring device for medium and small particle size monitoring the sowing process real time, so as to improve the intelligence level of the precision planter for rapeseed and wheat. In recent years, domestic and foreign scholars carried out many related studies on miss seeding and reseeding detection system mainly for potato, corn, wheat and other large and medium seeds. Few studies have focused on miss seeding and reseeding detection system for rapeseed and other small seeds because of its small size, light weight and high seeding frequency. Rapeseed and wheat are both seeding at high frequencies, but the rapeseed size is small(average particle size 0.8-2.2 mm), wheat size is larger(average length 6.25 mm, width 3.33 mm, thickness 3.07 mm), it is difficult for them to achieve compatible detection. In order to solve the above problems, a small and medium size seed flow monitoring device was designed based on the principle of photovoltaic effect of thin surface laser and silicon photocell in this paper. A thin surface laser emitting module with a thickness of about 1 mm and a photovoltaic effect produced by a silicon photocell were used. According to the emission angle of thin laser module and the diagonal length of silicon photocell, the size of seed monitoring area and the specific location of monitoring area were calculated, the structure parameters of the monitoring device, such as the inner diameter of the catheter, the position of the catheter center line, the relative position of thin laser module and silicon photocell, were defined. The time required for seeds to pass through the thin laser layer was analyzed. The response signal of rapeseed was completed within 3 ms and that of wheat seeds was completed within 7 ms. The response signal of seed crossing was transformed into single pulse signal by means of isolated direct traffic, double-stage amplification, half-wave rectification, voltage comparison and monostable trigger, and as an external interrupt source of single chip computer, thus the seeding information was counted, and the collision-free detection of small and medium sized seed flow was realized. Testing results of seed metering device metering at different revolving speed and counting instrument metering with high frequency showed that in the seeding frequency range of 8.4 to 32.1 Hz, the monitoring accuracy of rapeseed was not less than 98.1%, in the seeding frequency range of 21.5 to 31.2 Hz, the monitoring accuracy of wheat was not less than 95.1%. The results of field experiments showed that the device could monitor the seeding amount of small and medium sized seeds in real time, and the field light and vibration of machine had no effects on the monitoring accuracy.