For the problem of difficulty to guarantee products assembly process reliability, reliability-driven modeling approach of the assembly process was presented based on grey system theory and polychromatic sets theory. A method was described to determine the major assembly process step to public the reliability. Impact factors of each working step were analyzed. The methods to establish the relationship matrix between the various factors and the process step and to determine the main factors of the assembly reliability were elaborated. The control measures for reliability of the main factors were proposed. The purpose of reliability-driven for assembly process was achieved by above methods. An example of controlling assembly process for a machining center NC rotary table reliability-driven showed the feasibility of the method.