Five 166.6 MHz beta = 1 quarter-wave resonator (QWR) superconducting cavities (SC) have been chosen for the High Energy Photon Source (HEPS), a 6 GeV kilometer-scale diffraction-limited storage light source to be built in Beijing suburb. Each cavity is equipped with one high power input coupler to deliver over 180 kW RF power in continuous mode (cw). Overheating and the result in excessive heat load, even quench were observed during the horizontal test of the first proof-of principle (PoP) cavity. To find the overheating reason, a dedicated two-way coupled electromagnetic-fluid-thermal simulation of the helium gas-cooled coupler was carried out by Ansys solver. This paper presents some key parameters of the coupler calculated by the two-way coupled electromagnetic-fluid-thermal numerical simulation and the accuracy of the simulation was proved by comparing with the horizontal test results. Based on the electromagnetic-fluid-thermal coupling analysis, this paper takes into account the inlet gas temperature and flow rate on the temperature distribution, electromagnetic loss distribution, and 4.2 K heat load of the coupler. Furthermore, the occurrence conditions of overheating and quench are explored and some improvement suggestions of the coupler structure are given.