The origin of lock-in error of the mechanically dithered ring laser gyroscopes (MDRLG) is theoretically analyzed. It is obtained that the lock-in error relates to the input angular velocity, the dither frequency and the dither magnitude. According to the limitation of pulse count decoder caused by the readout circuit, a new method for eliminating the lock-in error of RLG is presented. At first, the phase character at the zero rate of crossing is described by a parabola equation. Secondly, the fractional angular signal that is smaller than a pulse count is compensated. Finally, by utilizing the values of sine, cosine and the second derivate of the phase at the zero rate of crossing, the lock-in error is eliminating. The simulation results show that the maximum output error of RLG is reduced from 6.25% to 0.622%. So this method improves the detection precision of RLG, and the output signal of RLG has preferably stability and repetition.