Processing of optical aspheric parts by the lapping tool of bending and forming is a new method and the processing is line contact. The lapping principle of the bending and forming method is given. Based on the Preston equation, the specific lapping conditions is put forward to the bending and forming method. Analysis is made about the function of the pressure and the relative velocity on the any point of at the processing surface. The material removal function is deduced about the bending and forming method, and the lapping time function is verified to be accurate. The lapping model is an important theory basis for the processing practice of the bending formation method.