We studied the effect of polarization of terahertz pulse and probe beam on terahertz detection in optically isotropic detection crystals using wave coupling theory of linear electro-optic effect. The study results show that the effect is remarkable, and the detection can get its largest sensitivity when the probe beam propagates along (110) direction which is most suitable for terahertz detection. Some of the study results are in good agreement with the previously published experimental results. So our study is helpful for the design of electro-optic detectors.