The choice of primary Lamb wave mode and the determination of excitation conditions play important roles in the nonlinear Lamb wave technology. The excitation window of the second-harmonic Lamb wave is defined by accumulation distance when the phase velocity is approximate matching. The strictness of the primary Lamb wave excitation condition is discussed. The excitation efficiency of the second-harmonic Lamb wave is evaluated by the amplitude of surface displacement. The influence of phase velocity approximate matching on the excitation efficiency as well as the excitation efficiency of different modes of Lamb wave is discussed. The theoretical results show that, within the accumulation distance, the increase of the propagation distance could improve the excitation efficiency of the second harmonic. But, the harmonic generation conditions are more strictly. A certain degree of phase velocity approximate matching will improve the excitation efficiency of second-harmonic Lamb wave. For different primary wave modes, the excitation efficiency and the strictness of the excitation condition of second-harmonic Lamb wave are different. The excitation window and the relative nonlinear coefficient of the longitudinal S2 / S4 mode at 90 and 150 mm propagation distance, as well as of longitudinal S1 / S2 and S2 / S4 mode at 150 mm propagation distance, are measured and compared. The measurement results are consistent with the theoretical analysis. In this article, the nonlinear effect of the Lamb wave in the case of approximate matching of phase velocity is discussed, which provides the analysis method and basis for the practical application of the Lamb wave.