When the signal is transmitted between systems with different modulation methods, the modulation format needs to be converted. Modulation format conversion is usually carried out on electrical signals and requires a process of electro-optical conversion after optical-electro conversion. Frequent optical-electro or electro-optical conversion usually increases the system cost. To address this problem, the implementation of format conversion from Binary Phase Shift Keying(BPSK) to Quadrature Phase Shift Keying(QPSK) and from QPSK to Quadrature Amplitude Modulation(16QAM) by using coherent superposition methods on the optical domain is proposed. In the experiment, the conversion model is simulated by Optisystem software, Taking 10 Gpbs pseudo-random signal as the test signal, it is successfully converted to QPSK and 16QAM observed by constellation diagrams. The Bit Error Rate(BER)of the conversion model and the direct QPSK and 16QAM transmission models are compared under different Optical Signal Noise Ratios(OSNRs), optical source bandwidths and signal powers. The BERs of the conversion model and the direct transmission remain highly consistent under the same conditions, indicating that this modulation format conversion method has high stability and accuracy, and can be applied to signal transmission between different systems.