In the satellite payload system, the multiplexer is mainly used to separate and combine the radiofrequency signals, which has a substantial impact on the performance of the whole system. With the good performance, compact design and easy implementation in the engineering, manifold multiplexer is commonly applied. In the traditional design method，software optimization technique is used, ideal response of the multiplexer is obtained by optimizing waveguide stub, the direct synthesis method is uncommon. A kind of direct synthesis method of filter with complex load is introduced, and applied in the design of manifold multiplexer. Through the power wave normalization theory, the objective polynomial of the filter network with complex load is calculated, then the coupling matrix of each channel filter is obtained, and the response of the multiplexer is obtained. The synthesis steps are listed by an example of triplexer. Finally, the response curves are obtained by simulation, which verified the feasibility of the method.