Abstract: The intake and drainage system of cooling water is a key component of coastal power plant projects. In recent years, as China's nuclear power development has steadily expanded and shifted toward offshore islands, increasingly complex wave conditions in open sea areas have made wave forces a dominant factor influencing the safety of drainage structures. Physical model tests were carried out to simultaneously measure forward, transverse, and uplift wave forces on a variable cross-section drainage riser structure with a steel cap. The time-varying characteristics of these forces were analyzed, and the effects of water discharge and submergence depth were examined. Calculation methods for the forward and transverse wave forces on the drainage riser were established. The results show that the peak forward and transverse forces of the steel cap riser change synchronously, while the uplift force peak exhibits a phase difference of about π/2 with the other two, unaffected by drainage. Water discharge significantly increases the uplift force on the riser, with the maximum relative uplift occurring at a relative submergence depth of approximately 0.013. By introducing a porosity reduction coefficient into the Morison equation, wave forces on porous structures can be calculated with an error below 4%. The wave force calculation method proposed here offers a reference for future engineering design and numerical simulations of similar structures.