Abstract: Planetary roller screw mechanisms are widely used in key fields such as aerospace, CNC equipment, and engineering machinery due to their high load-carrying capacity and long service life. However, the testing of full-scale prototypes for load-carrying characteristics and fatigue life is challenged by high costs and stringent requirements for test equipment. To address this issue, a scaling method based on equivalent reduced-scale modeling is proposed for planetary roller screw mechanisms. The validity of the method is verified by comparing the contact stresses of the prototype and the scaled model through simulation. The main research contents are as follows. Firstly, based on Hertzian elastoplastic contact theory and Buckingham's π theorem (the second similarity theorem), nine independent similarity criteria covering geometric, material, and dynamic characteristics are derived, and an equivalent load mapping relationship between the prototype and the scaled model is established. Meanwhile, a simplified meshing model of the roller and screw is constructed, and contact stress simulations are carried out under loads ranging from 20% to 100% of the rated load. The effectiveness of the scaled model is validated by analyzing the contact stress distribution on the screw and roller threads.