In order to investigate the round jet fragmentation mechanism of impact sprinkler, the primary breakup process of impact sprinkler round jet was simulated by using geometry reconstruction interface tracking method based on VOF multiphase flow model provided by Fluent, and the primary breakup length and crusher droplet diameter were obtained under different low pressure conditions from 200~ 600 kPa. The results were compared with the high speed camera technology, and relative error between numerical simulation and experiment results of the breakup length and primary crushing droplet diameter was analyzed. The results showed that the primary breakup shapes included continuous, transitional and fragmental sections. Nozzle diameter and the inlet pressure were the main factors affecting droplet diameter and breakup length. Primary crushing droplet diameter and the nozzle diameter d （ d >5 mm）had a good correlation ( D =1.634 d , R 2 =0.912), the relative error of droplet diameter between simulation and experiment was 23.92%. The fitted relationship between Weber number ( We ) and breakup length L could predict the sprinkler breakup length under low pressure conditions. As the increase of Weber number, the breakup length increased. The results had a good significance on the theory of water round jet spray dispersion and irrigation uniformity improvement.