Abstract: A loosening-preventing device for pipe fitting. In the device, a casing includes a semispherical member equipped with an open surface, a female screw thread disposed at one end of the semispherical member, and a finishing member disposed at the other end of the semispherical member. A locking member has a male screw thread and a head at one end of the male screw thread, so as to connect to the female screw thread of the casing. The screw clamping of the casing and the locking member prevents the pipe fitting from rotating, thereby preventing the pipe fitting from being loosened.