RET(reliability enhancement testing) is the most advanced technology in western countries. RET’s concept, process and stress limit classification were introduced. The basic theory of RET was explained. Interference model was introduced into RET, to settle the problem that RET couldn’t be analyzed quantitatively. The failure mechanism occurred in RET was analyzed by using interference model. Because the test stress was higher than the operation stress limit of product, almost all products in RET failed. By analyzing the interdependence between the operating stress found in RET and the stress in actual operation environment, the formula for reliable probability was deduced based on interference model. Then an example was given to explain this approach’s feasibility. RET could improve the reliability of products by improving the quality of design and manufacturing process. By using the interference model and the deuced formula, two ways of improving the reliability of products were analyzed. And also the basic principle of RET was explained based on the model.