Abstract: The electro-optic (EO) sampling method was employed for the coherent detection of a tunable high power FEL-THz source at Huazhong University of Science and Technology （HUST-TFEL）, due to its short response time, high sensitivity, low noise and wide bandwidth. Based on the theoretical analysis, the optimal polarization orientations of the femtosecond laser pulse and the THz pulse with respect to the EO crystal axis were obtained, and the phase match properties of the two laser pulses in the normal EO crystals (ZnTe, GaAs and GaP) were analyzed. Also the feasible crystal types and their optimal thicknesses were presented. The results provide useful guidance for the future detection debugging and experiment.