Geometric calibration errors in deflectometry are the main factors limiting the precision of low-order surface measurement. The relationship between the geometric calibration errors and low-order surface measurement errors is analyzed with mathematic model, theory simulation and experiment. The mathematic model for the relationship between the geometric calibration errors and the surface measurement errors is introduced and verified by simulations and experiments. The results show that the coordinate translational errors in geometric calibration introduce tilt and defocus into the surface measurement results. Besides, the longer distance between the tested mirror and the camera, display, the less effect the geometric calibration errors have on the low-order surface measurement. The results can help designing the system configuration of deflectometry and improving the accuracy of low-order surface measurement.