A device for measuring flow rate of wet gas based on an exempt radioactive source, includes a section of cylindrical pipe and a conical throttle located inside the cylindrical pipe and coaxially arranged therewith. The conical throttle includes a head cone section and a tail cone section arranged to have a common bottom surface. The head cone section faces a wet gas inlet of the cylindrical pipe. An annular gap is defined between the inner wall of the cylindrical pipe and the maximum diameter of the conical throttle for passage of wet gas. An exempt radioactive source block is arranged at the maximum diameter of the conical throttle in such a way that the gamma rays emitted from the radioactive source block can transmit radially through the annular gap to reach the gamma ray detector located outside the cylindrical pipe.