The present invention discloses a protective cap for a radio-frequency connector. The radio-frequency connector comprises a plug connecting piece and a socket connecting piece, wherein the protective cap can be removably mounted to the socket connecting piece for protection. The protective cap comprises a protection part structured to engage with the socket connecting piece and an inspection part structured to only allow a matched plug connecting piece to be inserted in. Hence, the inspection part can be used for inspecting the specific type of the plug connecting piece. The present invention further discloses an application method of the protective cap.