Fuzzy mathematics, reliability theory and multi-objective optimization technology were applied to design tractor’s final transmission with external single-stage spur gear based on comprehensive consideration on its loading capacity, structure, tooth face wear and transmission efficiency. The mathematical model for the multi-objective reliability optimization design was set up under the objective of the minimal volume and gears’ maximal sliding coefficients, and the maximal efficiency of the mechanism, with the design variable of module, teeth number of the pinion, teeth width and meshing angle. Then, the fuzzy solution of multi-objective optimization and genetic algorithm were used to solve this model. On the premise of ensuring the maximal sliding coefficient below the allowable value, the final transmission’s volume reduces by 1.55% and 6.74% of the common reliability optimization and original design scheme respectively, and the efficiency improves.