When the nozzle is decorated without offset and incline,gears with transmission ratios above 1 won’t be efficiently lubricated. In order to solve this problem,analytic method is adopted to establish mathematical model,basing on the geometric relationship between running gears and the jet flow. The mathematical model is used to calculate the impingement depth on aviation spur gears lubricated on the disengaging side. The correctness of the mathematical model is verified by simulation. The effects of gear parameters,transmission parameters and nozzle position on impingement depth of spur gear are analyzed. The results show that,the impingement depth decreases with the increasing of teeth number and rotating speed,while it increases with the increasing of jet velocity. The impingement depth on pinion is negatively correlated with that on gear as the change of jet flow offset and inclination angle. The research results provide theoretical bases for oil lubrication design.