Abstract: The high precision attitude measurement system is researched, which is made of a model-90 dithered ring laser gyroscope and quartz flexibility accelerometers, and a method to rotate the inertial measurement unit is developed. The hardware of the system is introduced and a mechanical design concept without damper is given. The technique of the alignment and the attitude computation are described. The alignment consists of coarse alignment and precise alignment, and system error propagation characteristics can be described by system error model. The static test and single-axial rotation test are performed. The results show that the precision of the attitude in 24 hours is better than 30 seconds, and the precision of the heading in 24 hours is less than 30 seconds.