In image measurement system, accurate focusing is critical for high precision measurement. For image focusing, a focusing evaluation function is introduced to verify whether the image is in focus or not. When the edge of a work does not lie in a plane perpendicular to the vertical direction, it is difficult to focus correctly with conventional focusing evaluation functions. So a focusing evaluation method is proposed based on the edge tracking. Firstly, the edge of interest is extracted with edge tracking. For each edge point, sums of gradient square within a certain area along the edge direction are calculated and the focusing evaluation function is the sum of the sums of gradient square. When the focusing evaluation function reaches its maximum, the edge is in focus. Experimental results show that, when the edge of a work does not lie in a plane perpendicular to the vertical direction, it can be focused correctly with the proposed method.