In order to solve the key technical problem in maintenance and repairing of heavy CNC machine tools, the concept of precision reliability and its forecasting module are proposed. The method is especially used for researching of reliability of heavy machine tools of which the samples are not enough for statistical methods. Firstly, spatial error module is established for a type of super heavy mill &boring machine tool based on multy-body kinematics. Then, the effect of wearing amount of kinematic pair of machine tools on machining precision is researched. Therefore forecasting modules for maintenance period and premature wearing of heavy CNC machine tools are built up. The method proposed can be used in calculating for maintenance period of heavy machine tools, analyzing for source of machining errors, and optimal design for total life of CNC machine tools. It can also be used in research of reliability for similar product of single piece or small batch.