In the quantum-classical signal simultaneous transmission scheme sharing a same few-mode fiber based on mode division multiplexing, the mode coupling effect in the optical fiber will lead to crosstalk between channels, resulting in bit error. In this paper, a segmented link model of few-mode fiber is constructed, and the magnitude of mode coupling strength due to fiber splicing error is studied, based on which the system quantum bit error rate (QBER) formula is deduced in the presence of mode coupling. By comparing the coupling strength of each mode, the optimal transmission mode of quantum signal in the scheme is determined. The effects of mismatch distance, torsion angle, and fiber length on the QBER are discussed. The results show that the three factors that affect the coupling strength of link mode are positively correlated with the QBER, and the quantum-classical signal simultaneous transmission system with small fiber splicing error in short distance can effectively reduce the QBER.