To improve the transmission performance of the radio over fiber system based on coherent detection, a scheme combining polar code with probabilistic shaping (PS) of many-to-one (MTO) mapping is proposed. The scheme uses many-to-one mapping to map 64 quadrature amplitude modulation (64QAM) signals to 44QAM signals to realize PS without extra PS redundancy; Polar code is used as forward error correction code scheme, which assigns the frozen bits of polar code to the overlapping bits of 44QAM. During decoding, the overlapping bits are implicitly pierced by successive cancellation list decoding, so that the receiver can receive the signal correctly and improve the reliability of transmission of the system. The simulation results show that at a symbol rate of 10 Gbaud, the bit error rate of the system can be increased from 2.52×10-2 to 1×10-3 compared with that of the traditional uniform (UD) signal. When lg(R_BER) =-3 and the fiber length is 20 km, the received optical power of the Polar-MTO-PS signal is 2.6 dB lower than that of Polar-UD signal, and 4.6 dB lower than that of the low density parity code (LDPC)-MTO-PS signal. The optical signal-to-noise ratio of the Polar-MTO-PS signal is 3.3 dB better than that of the LDPC-MTO-PS signal.