Based on theoretical equation of laser gyro beat output, scale factor forward-reversal rotation asymmetry of rate biased laser gyro is analyzed by theory, simulation, and experiment. Since the scale factor forward-reversal rotation asymmetry highly affects the precision of rate biased laser gyro, the relation between scale factor and rotation direction is analyzed by theory and numerical simulation, respectively, and a new kind way to measure the scale factor forward-reversal asymmetry is provided without knowing the laser gyro zero drift and earth rotation component. The experiments show that the scale factor forward-reversal rotation asymmetry of a laser gyro is approximately 0.02×10-6 with rotation rate of 72°/s.