It is difficult to apply a traditional strapdown inertial navigation system(SINS) to spinning projectile because of high spin rate. A Roll-Isolated gimbal platform is introduced to prevent SINS by the projectile’s spinning motion. Roll-Isolation is accomplished by supporting the inertial measurement unit(IMU) on a single gimbal, the axis of which is parallel to the projectile’s spinning axis. Roll-Isolation prevents the saturation of the roll gyro by the high vehicle spin rate and greatly reduces the measurement errors arising from gyro scale factor and misalignment. On the basis of roll-isolation, the paper presents a Two-Position initial alignment scheme of SINS on stationary base, which is realized by changing the IMU roll angle around the spinning axis. Furthermore, the paper studies on the observability of the scheme, and the results show that it not only improves observability but also minimizes alignment errors.