The sensitivity of different modes to cracks at different locations are different. In response to the problem of inaccurate mode selection when detecting cracks, the selection of optimal crack-sensitive mode ( SOCSM) method evaluated by three indicators is proposed, which considers the effects of cumulative second harmonic modes combination, mode vibration pattern and crack location under 50 kHz excitation. The sensitivity of modes to cracks is evaluated by the trend of relative nonlinear coefficients with crack growth. The optimal mode excitation (OME) algorithm is proposed to accurately excite the required modes. To evaluate the effectiveness and accuracy of the SOCSM and OME methods, numerical simulation analysis and experimental verification are conducted by using the crack at the waist of the CHN60 rail as an example. The results show that the OME method can accurately excite mode combination 11 and mode combination 1. Under 50 kHz excitation, the mode combination 11 β monotonically increases with crack growth, while mode combination 1 β has almost no change. The mode combination 11 β is more suitable for detecting cracks at the rail waist and is very sensitive to small changes in cracks. Both modal combinations can generate cumulative second harmonic. The presence of cracks can affect the propagation of guided waves, which affects the amplitude of the signal.