The invention relates to an online monitoring method for residual pretightening force, in particular to an online monitoring method for residual pretightening force of a large-sized prestressed steel wire winding frame. The online monitoring method comprises the steps of determining a measurement area of the frame and treating the surface of the measurement area; before a steel wire is wound, welding a weldable strain gage on the frame; connecting a data line of the strain gage with a strain data acquisition instrument and connecting the strain data acquisition instrument with a computer by using a data line; acquiring and storing initial data of the strain gage; during working after the steel wire is wound on the frame, acquiring the strain data of the strain gage again, making a comparison between the strain data which are acquired at two times to obtain the strain of the frame, and calculating the residual pretightening force according to the strain. The online monitoring method for the residual pretightening force of the large-sized prestressed steel wire winding frame solves the problem that the residual pretightening force of the large-sized prestressed steel wire winding frame during working cannot be quantitatively monitored.