This paper uses Flexsim software to do a simulation on American system straight tap processing and assembly line, analyzes the simulation results, finds out the bottlenecks of the production system, and propose feasible improvement solutions. And then it uses the Flexsim modeling again, to improve the program for the second and third time, get the final improvement plan, and compare simulation results before and after improvement, verify the feasibility of the improvement plan. It will provide a reference for the improvement of production systems in M plant. 