A systematic design method of the laser welding process is proposed. A simplified finite element simulation model for the single-fiber bidirectional optical subassembly (BOSA) is established by using the ANSYS software. The numerical simulation method of the indirect welding stress field is adopted to study the parameters which render the minimum offset under the single beam laser welding. By combining the optimized parameters, the optical power losses after welding are analyzed under different laser welding machine parameters. The results show that when the single beam laser power is 100 W, the pulse width is 0.2 ms, and the spot radius is about 0.18 mm, the optical assembly has the minimum offset and the optical power loss ratio after optimization is decreased by 7.1%.