Embodiments of the present invention relate to the optical communication field and disclose an optical signal multiplexing method and an optical multiplexer. The method provided in the present invention includes: adjusting polarization states of two of four optical signals to be multiplexed, and preferably, mutually orthogonal to, the polarization states of the remaining two optical signals; combining one optical signal in the adjusted polarization state with one optical signal in the unadjusted polarization state into one optical signal through polarization multiplexing; and combining the two optical signals obtained through polarization multiplexing into one optical signal. When multiplexing is performed according to the solutions of the present invention, all optical signals are multiplexed into one optical signal based on polarization multiplexing, optical paths traveled by different optical signals differ slightly, and the optical power and the optical field energy distribution differ slightly between different optical signals in the finally multiplexed optical signal.