Incorporating the error into the tooth stiffness model of spur gear, the variety law of gear stiffness was obtained based on material mechanics method, and the effects of the error on the gear stiffness were analyzed. The mechanical method was employed to compute the tooth stiffness instantaneously. The tooth stiffness was calculated as an effective function, which also include the effect of error by the difference of the y-axis. The method was illustrated via two error examples, and the effects of the error on the stiffness of different plots on the tooth were examined analytically. The results show that the effects of the error on the stiffness increases with the increase of error, the influences of the error on the dedendum stiffness is more evident than that on the addendum, and the influences on stiffness of the driving gear is greater than that of the driven gear under the same error.