The wavelength of the probe laser can affect the bandwidth and the efficiency of terahertz electro-optic sampling (EOS) detection method because of the relationship of velocity match between the terahertz phase velocity and the laser group velocity. An electro-optic (EO) crystal has different dispersion with different lasers, thus the wavelength of the probe laser for a given EO crystals have different influence on the EOS detection. The response functions of EOS systems based on two typical EO crystals, ZnTe and GaP, have been studied. The results show that the probe laser with wavelengths of 800 nm and 1200 nm are more suitable for the ZnTe-and GaP-based EOS system, respectively, than other selected wavelengths (600 nm, 800 nm, 1200 nm and 1600 nm). We also found that for a given thickness of an EO crystal, there exists an optimal wavelength for the probe laser pulse to obtain the broadest response function with high value.