Described are a test apparatus and a test method for the wetted perimeter of coal seam water injection. In the test apparatus, a columnar insulator is provided between an upper electrode and a lower electrode, circular insulating tapes are located at the outer edges of the upper electrode and the lower electrode, a circular reverse osmosis membrane is provided at the middle of the circular insulating tape, the upper electrode, lower electrode, circular insulating tapes and circular reverse osmosis membrane form an enclosed chamber which is filled with solid sodium chloride, and cotton yarns are packed among the upper resin backing plate, lower resin backing plate, circular insulating tapes and the inner walls of water permeable perforated pipes. The upper electrode is provided with an electrode lead which passes through the columnar insulator, the lower electrode and the lower resin backing plate and goes out from the tail connecting end.