Hefei Advanced Light Facility (HALF) will use a 499.8[Formula: see text]MHz superconducting cavity to provide energy for the beam in the storage ring. The fundamental power coupler (FPC) needs to feed 140[Formula: see text]kW RF power into the superconducting cavity. In order to meet the requirements of the HALF project, a 499.8[Formula: see text]MHz FPC prototype was developed at the National Synchrotron Radiation Laboratory (NSRL). This paper presents the design of the FPC prototype. The fabrication process of the FPC prototype will also be described. The cold test of the FPC prototype room-temperature test stand was carried out. The results show that the cold test performance of the FPC prototype meets the requirements and it can be tested with high power. The high power test of the FPC prototype will be carried out in the future.