The invention provides a friction-stir welding spindle constant displacement control method and device and belongs to the technical field of welding. The control method comprises the steps of (1) controlling the constant displacement of a welding spindle so as to conduct friction-stir welding when receiving a welding control instruction; (2) acquiring the deformation displacement of the position, opposite to the welding spindle, of a welding spindle supporting mechanism and/or a workpiece supporting mechanism; (3) determining the axial follow-up interpolation displacement of the welding spindle according to the deformation displacement; and (4) controlling the axial interpolation displacement of the welding spindle according to the axial follow-up interpolation displacement. According to the method, the problem that the actual distance from the tail end of a stirring head to the workpiece supporting mechanism cannot be kept consistent due to deformation of the welding spindle supporting mechanism and/or the workpiece supporting mechanism is effectively avoided by acquiring the deformation displacement of the welding spindle supporting mechanism and/or the workpiece supporting mechanism and controlling the axial interpolation displacement of the welding spindle according to the deformation displacement; and the welding quality of roots of joints is guaranteed.