Abstract: Based on the consideration of various characteristics of the double tansmission reducer, in pursuit of the goal to enhance the overall performance and comprehensive economic benefits of the system, a method for transmission ratio distribution is presented, taking into account three factors: minimum volume, minimum center distance, and equal strength calculation. Taking a double tansmission helical gear structure as an example, the impact of transmission ratios on the dimensions of the reducer and the safety factor of the gears is analyzed. Increasing the first-stage transmission ratio could reduce the weight of the reducer; When the transmission ratios are distributed according to the principle of minimizing weight (volume), the resulting reducer has relatively smaller dimensions within the vertical plane of the shaft. As the first-stage transmission ratio increases, the difference in contact stress in the double gear tansmission reducer also increases. The impact of transmission ratio on the minimum safety factor S_H for tooth surface contact is relatively minor, but it has a significant effect on the minimum safety factor S_F for root bending.