Discrete calibration is a widely used calibration method because of its principle's simplicity. However the calibration result may suffer errors for its limitations on precision. In this article, the roots of the calibration error equations of FOG are calculated firstly, and the effect of calibration errors to the precision of AHRS is analysed. Secondly, a series of simulations are run in order to observe the effect of the calibration to the precision of AHRS. Finally, the simulation results are correspondent to the theoretical calculation, which thus proves the correctness of theoretical deduction.