The spectral resolved femtosecond pump-probe experiment is applied to measure the third order nonlinear refractive index coefficient, two-photon absorption coefficient of GaP crystal, and the chirp parameter of the input laser pulses. The results show that nonlinear refractive index coefficient is 2*10^(-18) m2/W, two-photon absorption coefficient is 2*10^(-11) m/W at wavelength of 783.5 nm, and the chirp parameter of laser pulse is 1*10^(25) s^(-2). Furthermore, the mechanism of nonlinear refraction due to two-photon absorption in GaP crystal and experimental results are discussed.