There are many coupling loss factors between optical devices and input/output fibers, mainly including temperature, stress, mode field mismatch, etc., which cause additional losses. The Multimode Star Coupler (MSC) module in the optical control converter valve of the power system is considered as an example with simplified quantitative analysis, it is found that the influence of thermal expansion on the coupling loss between the fiber array and the waveguide array can be ignored. Based on waveguide theory and mode coupling theory, the influence of input and output waveguide structure of MSC module on coupling performance was analyzed using numerical calculation methods-the mode field area and mode mismatch between waveguide and fiber caused significant coupling losses. Therefore, an inverted taper structure, transforming the mode field size and reducing the mode mismatch between the waveguide and the fiber, is optimized to minimize the coupling loss between the waveguide and the fiber.