The invention discloses an adjustable positioning device used for lathe rear end positioning and machining. Workpieces of various specifications can be positioned by adjusting. The adjustable positioning device comprises a positioning interface located between three jaws of a lathe and fixed with a screw; an inner hole of the positioning interface and a spindle of the lathe are concentric; a positioning tube sleeve is fixed in the inner hole of the positioning interface in the axial direction; the hole diameter of the positioning tube sleeve is smaller than or equal to the hole diameter of theinner hole of the positioning interface; the positioning tube sleeve is used for backwards extending into a spindle box of the lathe; a positioning plug block is fixed to the rear end of the positioning tube sleeve to seal an opening of the rear end of the positioning tube sleeve; a positioning block is arranged inside the positioning tube sleeve and used for axially positioning the workpieces; the positioning block is in sliding fit with the positioning tube sleeve; a connection rod which extends backwards in the axial direction is fixed to the rear end of the positioning block; the connection rod backwards penetrates through the positioning plug block; the positioning plug block is provided with a via hole to be in clearance fit with the connection rod; and a fastening screw is arrangedon the positioning plug block and used for locking the axial position of the connection rod.