Reverse Engineering (RE), as the important technology for learning and mastering the advanced technology, as well as shortening the period of product design and development, has already become the highlight in the manufacturing industry. But there are some inner un-measurable parts, like couplers. The inner form information of the coupler can not be fully captured without any cutting, which is one of the bottlenecks in RE. Therefore, an effective method needs to be explored for reconstructing lumen model. Taking model reconstruction of coupler lumen as a case, an assembly reverse method is proposed by analyzing the parts functions and restraint relations in coupler. The method in detail is that, firstly, all parts are assembled in the coupler body lumen, then, according to assembly relations and the enveloping space determined by movement scope of individual parts, the coupler lumen shape can be obtained. The method proposed is successfully applied in model reconstruction of heavy haul coupler, and which provides a common solution of model reconstruction for inner un-measurable parts.