The invention relates to an assembly method of a transmission shaft structure, and a screw tailstock is movably assembled and is self-aligned. A traditional screw tailstock is assembled in a mode that the center height of the screw tailstock is adjusted by adopting an adjusting pad and is fastened radially by using a bolt. Measurement and match grinding are needed, therefore, the skill requirement of assembly personnel is high, and the measurement and match grinding errors exist. According to the assembly method, by utilizing the characteristic that the position of a screw is limited by a screw nut when the screw nut moves to the tail end of the screw, the self alignment of the screw tailstock which is installed on the tail end of the screw is guaranteed, and the screw tailstock is axially fixed at the moment, so that the coaxial precision of the assembly position of the screw tailstock and the screw is guaranteed.