The invention relates to a thread trimming device with a protective cover body. The device comprises a trimming base, wherein a power source connector is disposed on the trimming base; a drive motor is firmly disposed on the trimming base; a drive rotation shaft is disposed on the top of the drive motor; a positioning screw is disposed on the top of the drive rotation shaft; and a trimming head shaped like a cone is firmly connected to the upper part of the positioning screw. The thread trimming device also comprises the cover body which is disposed outside the trimming head, wherein trimming space which can be used to store the trimming head is formed through enclosing of the cover body; and the area of the cross section of the trimming space decreases gradually from top to bottom. According to the invention, the protective cover body is disposed outside the trimming head, so that an operator can be protected from being hurt by splashed burr during trimming.