In order to realize the operating state diagnosis of gearbox in the SD16 style bulldozer rapidly,the laser particle size analysis method was used to monitor the number and size of abrasive particles in the gearbox oils.The particle size distribution of the abrasive particles in the oils at different time was obtained,and the working status of the gearbox was judged based on the test data.The growth laws of the maximum and minimum abrasive particles in the oils were studied to judge the abnormal wear time of device and achieve timely repair and maintenance of equipment.The results show that the laser particle size analysis method can monitor the particle size distribution of abrasive particles in the oils quickly and efficiently，which provides basis for judging the running state of the bulldozer’s gearbox and preventing equipment failure effectively.