In this paper, a new method for edge checkout—Laser Half-focus locating method is introduced. Many comprehensive techniques have been used to ensure high precision of edge checkout, such as self adaptive tracking technique for compensation, laser modulating and modulated amplitude compensating technique, fixed point sampling technique through differential and fine dividing technique for step motor. A theoratical model for edge checkout using laser half-focus and its result of computer simulating are also given. In the end, a system of measuring diameter adopting this technology is described.