Scale factor of rate biased laser gyro is precisely measured by calibrating fix-error angle. Common scale factor measurement methods are analyzed according to the characteristics of rate biased laser gyro structure,and it is found that the precision of measuring scale factor is determined by fix-error angle. A method of calibrating fix-error angle is put forward. With this method rate biased laser gyro is installed on position turntable by approximate 45°,the ouput pulses of four different states are measured,then the laser gyro fix-error angle can be calculated,and the formula of measuring scale factor is corrected by the fix-error angle. The experimental result shows that precision of measuring rate biased laser gyro scale factor can be higher than 10-7.