The study found that the slope bias of accelerometer would decrease the accuracy of Kalman filtering precision alignment or in-movement alignment of SINS. Especially for the SINS using quartz flexible accelerometer, the initial alignment would have a big error when the SINS was not preheated enough. This paper analyzes the concrete influence of inertial device’s slope error on the initial alignment of SINS, and proposed that to obtain high heading alignment accuracy, in addition to small equivalent east gyro drift, the equivalent north accelerometer zero bias must be stable enough during alignment. The slope bias of accelerometer was modeled and compensated through test, and the results showed that the heading alignment accuracy was improved obviously.