An electric assistant motor module for a bicycle, comprising: a shell; a motor fixed in the shell and having a rotor; a reducing structure comprising a wave generator, a circular spline, and a flex spline, wherein the wave generator has an input end; the rotor transmits torque to the input end; the circular spline is fixed in the shell; the flex spline has a flexible wall, wherein an outer peripheral surface of the flexible wall has a teeth portion; the teeth portion meshes with the circular spline; the wave generator abuts against an inner side of the flexible wall; a front-end edge of the flexible wall is connected to a ring disk which extends inwards; an inner peripheral edge of the ring disk forms a connecting base, wherein the connecting base has a threaded section and a friction surface; the threaded section of the connecting base is located on an inner peripheral surface or an outer peripheral surface of the connecting base and is connected to the friction surface of the connecting base; a mating part which is annular and is coaxial with the flex spline and has a fixing portion, wherein the fixing portion has a threaded section and a friction surface; the friction surface of the fixing portion is connected to the threaded section of the fixing portion; the threaded section of the fixing portion is engaged with the threaded section of the connecting base, and the friction surface of the fixing portion abuts against the friction surface of the connecting base; a crank spindle penetrating through the shell, the mating part, the reducing structure, and the rotor in a coaxial and rotatable way, wherein two ends of the crank spindle respectively penetrate out of the shell and respectively have a connecting portion; and a crankset fitting around a periphery of the crank spindle in a coaxial and rotatable way, wherein a first one-way ratchet structure is provided between the crankset and the mating part, and a second one-way ratchet structure is provided between the crankset and the crank spindle; the crankset has an engaging cylinder, wherein the engaging cylinder penetrates through the shell for being engaged with a chainring of a bike. a shell; a motor fixed in the shell and having a rotor; a reducing structure comprising a wave generator, a circular spline, and a flex spline, wherein the wave generator has an input end; the rotor transmits torque to the input end; the circular spline is fixed in the shell; the flex spline has a flexible wall, wherein an outer peripheral surface of the flexible wall has a teeth portion; the teeth portion meshes with the circular spline; the wave generator abuts against an inner side of the flexible wall; a front-end edge of the flexible wall is connected to a ring disk which extends inwards; an inner peripheral edge of the ring disk forms a connecting base, wherein the connecting base has a threaded section and a friction surface; the threaded section of the connecting base is located on an inner peripheral surface or an outer peripheral surface of the connecting base and is connected to the friction surface of the connecting base; a mating part which is annular and is coaxial with the flex spline and has a fixing portion, wherein the fixing portion has a threaded section and a friction surface; the friction surface of the fixing portion is connected to the threaded section of the fixing portion; the threaded section of the fixing portion is engaged with the threaded section of the connecting base, and the friction surface of the fixing portion abuts against the friction surface of the connecting base; a crank spindle penetrating through the shell, the mating part, the reducing structure, and the rotor in a coaxial and rotatable way, wherein two ends of the crank spindle respectively penetrate out of the shell and respectively have a connecting portion; and a crankset fitting around a periphery of the crank spindle in a coaxial and rotatable way, wherein a first one-way ratchet structure is provided between the crankset and the mating part, and a second one-way ratchet structure is provided between the crankset and the crank spindle; the crankset has an engaging cylinder, wherein the engaging cylinder penetrates through the shell for being engaged with a chainring of a bike.