Abstract： To improve the design quality of the non-symmetric double pressure angle gear and shorten the design cycle time, in accordance with elastohydrodynamic lubrication theory, by example, the authors took the maximum value of the smallest film thickness between gear teeth and the minimum value of the total volume of the transmission gears as the object function. And, constrained multi-objective optimization was achieved according to particle swarm optimization, programming with MATLAB. On this basis, study from the point of mathematical logic and physical mechanism were carried out for the sensitivity analysis of objective function to each design variable according to gear engagement theory and principles of modern tribology. The results show that with module and width of gear increasing, the volume of the meshing non-symmetrical gears increases, and it is more sensitive to module comparing with width of gears, the film minimum thickness increases as each of module, width of gear, pressure angle and modification coefficient increases. Pressure angle, module, width of gear and modification coefficient are in order of the degree of sensitivity. Especially, pressure angle is the most important factor for the elastohydrodynamic lubrication film minimum thickness between gear teeth. The increase of the pressure angle on the working profile significantly causes rising of smallest film thickness. Coefficients of all the meshing gears, no matter big or small, have equal influence on the film minimum thickness．