A transmission mechanism of a spinning fishing reel includes a transmission shaft having a shaft body with an axial hole and a cut slit communicating with the axial hole, a gear disc having a tube portion, through which the shaft body is inserted such that the gear disc is drivenable by the transmission shaft to rotate, and a bolt having a head and a shank. The shank is screwingly threaded into the axial hole in a way that the head of the blot is abutted against an end of the tube portion of the gear disc, such that the outer periphery of the shaft body is expanded to firmly abut against the inner periphery of the tube portion. As such, the operational smoothness and stability of the product are enhanced by the transmission mechanism. Further, the transmission mechanism can be easily assembled with low manufacturing cost.