In the primer tightening process of a certain type of small bore ammunition automatic production line, the pre-tightening force is not easy to measure directly, the friction state fluctuates obviously, and the traditional torque method has a large dispersion. The sources of pre-tightening force fluctuation of traditional torque method were analyzed, and a 2-stage model of torque-angle method and the formation mechanism of pre-tightening force were established. A control scheme including fitting torque, target angle, final torque alarm and curve slope window was designed, and a 4-stage sub-tightening process was proposed. A parametric simulation model was established, and the Monte Carlo method was used to compare the two control strategies. The results show that the standard deviation of pretightening force is reduced from 227 to 94 N (58.5%), the process capability index is increased from 0.645 to 1.581, and the qualified rate is increased from 95.3% to 100.0%. The sensitivity analysis verifies that the torque-angle method has significant anti-disturbance advantage for friction fluctuation.