The invention discloses an on-off valve for preventing liquid return in a pipe string. The on-off valve for preventing the liquid return in the pipe string is used for preventing the liquid in a wellfrom returning out of a well head during the operation of an oil and gas underground pipe string, and comprises a central pipe, a connecting sleeve, a sealing ring, a reverse ball seat body, an upperbaffle ring, friction blocks, springs, a lower baffle ring, a ball and a lower ball seat. The central pipe is provided with a boss which is connected with the connecting sleeve in a hanging mode. Thelower end of the connecting sleeve is provided with a thread to be connected with the reverse ball seat body, the central pipe can reciprocate in the connecting sleeve and the reverse ball seat body,and the lower end of the central pipe is sealed with the reverse ball seat body through the sealing ring. The springs are arranged between the friction blocks and the reverse ball seat main body, andthe upper and lower baffle rings install the friction blocks and the springs on the reverse ball seat body. The lower baffle ring is provided with a thread to be connected with the reverse ball seat body. A lower end thread of the reverse ball seat body is connected with an upper end thread of the lower ball seat. The ball is placed between the reverse ball seat body and the lower ball seat to move up and down. According to the on-off valve for preventing the liquid return in the pipe string, a channel can be closed when the pipe string is lowered, the channel in the pipe string is opened whenthe pipe string is stationary, and the internal and external liquid levels of the pipe string can be automatically balanced; and when lifting or pulling up the pipe string, the forward and backward washing channels are opened to realize forward and backward well washing.