The simultaneous transmission sharing a same few-mode fiber of classical and quantum signals can be realized by using mode division multiplexing. In order to analyze the quantum signal influenced by classical optical signals due to optical fiber nonlinear effects during transmission, the relationships between the crosstalk power caused by nonlinear effect and fiber length, as well as the power of classical optical signals are investigated, and a quantum bit error rate model under the influence of nonlinear effect is constructed. The simulation analysis is carried out for the three modes optical fiber system, and the effects of fiber length and classical optical signal power on the quantum bit error rate are discussed. Simulation results show that when the signal wavelength is 1550 nm and the power of the classical optical signal is －10 dBm, the quantum safe communication distance can reach 155.7 km, among which the power of the classical optical signal is the biggest factor influencing the safe communication distance.