Abstract: During the operation of a ball screw pair, frictional heating between balls and raceways causes thermal deformation, which exacerbates uneven load distribution and significantly reduces transmission accuracy and fatigue life. To address this issue, a load equalization design method for ball screw pairs based on variable-diameter balls is proposed. A non-standard ball diameter adjustment strategy is adopted to optimize the contact stress distribution under combined thermal and mechanical loads by adjusting the diameter of heavily loaded balls. Specifically, a variable-diameter ball screw pair is established, and a two-dimensional finite element model under multi-physical fields is constructed to quantitatively analyze the load distribution under axial load and temperature rise. Photoelastic experiment technology is used to invert the contact stress distribution via isochromatic fringe density, realizing stress visualization in the contact area. Experimental and simulation results confirm that the design significantly improves load distribution uniformity at steady-state operating temperature, and the established theoretical model and design method provide key support for high-precision ball screw pair design.