The mechanical dither ring laser gyro (MDRLG) can achieve high accuracy. But the MDRLG must go across lock-in region twice every dither cycle, thus the information loss is inevitable. Random noise injection in the dither amplitude can only cancel the accumulation of lock-in error, but not the error itself. According to the lock-in equation, the side effect of lock-in phenomenon is interpreted in detail. Theoretical research on lock-in error compensation for MDRLG is carried out. Taking the constant acceleration approximation in lock-in region, the lock-in error representation is obtained. Numerical simulation results show that even in different input angle rates, the compensation is also very effective.