A method for extracting a gear tooth profile edge based on an engagement-pixel image edge tracking method includes defining a transmission ratio relationship between a cutter and an envelope tooth profile, setting a cutter profile step size and an envelope step size, acquiring instantaneous contact images at different engaging times, and performing a binarization processing on each curve envelope cluster image; sweeping a boundary of an envelope curve cluster, acquiring pixel points of the edge; preliminarily tracking a tooth profile edge, and then performing a secondary extraction and compensation on the pixel points; calibrating coordinates of a cutter profile; extracting a pixel coordinate of an instantaneous engaging point; converting the pixel points among different instantaneous engaging images; extracting a final tooth profile coordinate of the gear, and performing a tooth shape error analysis and a contact line error analysis.