In order to reduce the cost of the laser guidance weapons system and improve its accuracy, the strap-down laser guided seeker was designed. The precision of angle measurement which affecting the attack precision was studied. First, the method of gain controlling affecting the precision of angle measurement was analyzed. Second, the four channel variable gain amplifier was calibrated and the least squares fitting curve of the gain was got. Then, the method of gain compensation and its error under the discrete control were discussed. Finally, the error of light spot between uncompensated and compensated in different amplified mode by simulation was got. Experimental results indicate that the system error is decreased by 5.6 mrad. The precision of angle measurement reach to 2 mrad in the linear field of view. It can satisfy the system requirements by semi-active laser guidance weapons system.