A method of testing dynamic performances for a slider body of a thermally-assisted magnetic head includes: providing a slider body which is disconnected with a light source unit; removably mounting the slider body to a test head suspension assembly; keeping to provide a flat top beam to the slider body, the flat top beam being aligned with the optical waveguide, and a projection of an incident end of the optical waveguide being located within a light spot of the flat top beam; and testing the dynamic performance of the slider body. It can save the material cost and labor cost, and eliminate a precise optical alignment between an input light and an optical waveguide in the slider body to improve testing efficiency.