In order to build a two-stage helical gearbox (THG) with first stage double gear-sets (FSDG), the multi-criteria decision-making (MCDM) method is introduced in this research as a new approach to solving the multi-objective optimization problem (MOOP). The study's objective is to deter-mine the best primary design factors that simultaneously satisfy two goals: maximal gearbox ef-ficiency and minimal gearbox mass. To that end, the first stage's gear ratio and the first and sec-ond stages' CWFW were chosen as the three main design elements. Furthermore, two distinct goals were analyzed: the lowest gearbox mass and the highest gearbox efficiency. Additionally, the MOOP is carried out in two steps: phase 1 solves the single-objective optimization problem to close the gap between variable levels, and phase 2 solves the MOOP to determine the optimal primary design factors. Furthermore, the TOPSIS approach was selected to address the MOOP problem. For the first time, an MCDM technique is used to solve the MOOP of a helical gearbox with FSDG. Additionally, this is the first investigation of the total gearbox efficiency, which includes the power losses during idle. When designing the gearbox, the optimal values for three crucial design parameters were ascertained using the study's results.