Abstract: For study of the 90-type mechanically dithered ring laser gyro SINS system which is self-developed by the authors' department, this article detailed analyzed the convergence time and alignment precision of inertial alignment on static base and swing base. Theoretical analysis shows, the value of the alignment time should basis on the IMU accuracy, settings of the algorithm parameter and the disturbance situation. Through a large number of experiment, on static base, it takes 150 s for the heading angle reaches a steady state value, and then the oscillation amplitude of the roll angle and pitch angle are lower than 1.5, the heading angle oscillating is within 1'; on swing base, after 200 s, the error of the roll angle and pitch angle are lower than 7, the error of heading angle is within 1.5'. Results confirm that the inertial alignment can meet the requirement of reaching a certain attitude angle accuracy range within a short time, and can be well used in engineering applications.