The invention provides a method for post-treating ultra-high molecular weight polyethylene fiber, which comprises the steps of: a) imposing 5 to 60 times of drawing on dried ultra-high molecular weight polyethylene gel protofilaments at a temperature of between 70 and 160 DEG C to obtain the ultra-high molecular weight polyethylene fiber; and b) imposing the negative stretch with the drawing ratio less than 1 time on the ultra-high molecular weight polyethylene fiber at a temperature of between 100 and 160 DEG C. After the ultra-high molecular weight polyethylene fiber is subjected to ultra-thermal stretch, the fiber has ultra-tensile deformation, and the inside of the fiber can produce internal stress, so the method, by making the fiber perform certain thermal contraction, can effectively release the internal stress of the fiber, and improve the mechanical properties of the ultra-high molecular weight polyethylene fiber.