The dithering of mechanically dithered ring laser gyro (DRLG) leads to pseudo coning motion in the strap-down inertial navigation system (SINS). The influence of the pseudo coning motion on calculation of the rotating vector of SINS is analyzed, and the formula is deduced. To minimize the pseudo coning error, the influcence factors are analyzed by numerical calculation, and the matching principle of dithering frequency difference among different DRLGs in SINS is recommended. Based on the principle, a dithering frequency tunable technology for the practical difficulty during DRLG production is proposed and investigated numerically and experimentally. The results show that the proposed technique can meet the dithering frequecy difference matching demand of SINS by choosing one main dither motor plus appropriate adjusting components, and there is no negative impact on the overall size and the anti-vibration performace of DRLG. Compared to the conventional programs, the proposed technique promotes the productive capacity of DRLG, especially for the miniature DRLG, and reduces the production cost.