Aiming at the problems in the operation of small square baling machine, such as baling continuous, difficult dynamic separating and weighing a single bale, frequently rising and lifting of pickup, and ground up and down, a dynamic weighing system of square baling machine was designed based on multisensor fusion technology. A dynamic separation and identification method of bales was proposed by changing the track. A dynamic weighing system of straw bale based on realtime downforce and pitch angle of weighing platform was established. As a result, the dynamic independent weighing of single straw bale was realized. The system included mechanical part, sensor part, data acquisition module, satellite positioning module, display terminal and upper computer software. The sensor part includes four pressuretype weighing sensors, the tilt angle sensor of the weighing platform and the straw bale status identification sensor. The system could display realtime information such as pressure under weighing platform, pitch angle of weighing platform, bale weight, bale status, longitude and latitude, etc. In June 2020, the performance test of straw bale dynamic weighing system was carried out in Xiaotangshan National Precision Agriculture Research Base in Changping District, Beijing City. The results showed that the maximum relative error of the predicted bale weight in the static mode was 0.38%. In the dynamic mode, the R2 of the predicted bale weight and the true value reached 0.996, and the relative error range of the system predicted weight was -4.40%~4.30%. The results showed that the system had excellent accuracy and robustness, and the system could satisfy the practical needs of weighing grass bales in the field. The system provided a fast measurement method for the operation quality evaluation of baler.