The invention discloses a conversion valve device of an on-line mixing injection molding machine. The conversion valve device comprises a valve body and a valve element, wherein the valve element is located in the valve body. The conversion valve device is characterized in that the valve body is provided with a first valve body passage, a second valve body passage, a third valve body passage and a fourth valve body passage which are communicated with an extrusion machine, an injection cylinder, a pressure maintaining cylinder and a jet nozzle in the on-line mixing injection molding machine in a one-to-one correspondence mode; a metering passage set, an injection passage set and a pressure maintaining passage set which are distributed at intervals in the axial direction and are mutually independent are arranged in the valve element; the valve body is provided with a valve element movement driving mechanism. The conversion valve device has the advantages that the valve element is provided with the pressure maintaining passage, so that materials are fed into the injection cylinder in the pressure maintaining stage; compared with a traditional on-line mixing injection molding machine, under the condition of the same runner size, by using the conversion valve device, the formation period of the on-line mixing injection molding machine can be shortened, and the production efficiency can be improved by 10-20%.