Objective: To avoid the virtual tolerance phenomenon in the process dimension conversion, and improve the operability of machining process specification by processing and improving of machining process dimension chain. Methods: Based on the tolerance distribution relationship and error compensation relationship between the concluding link and each component link in the calculation of process dimension chain, studie the causes of virtual tolerance of process dimension and the improvement method of process dimension chain under the condition that the part process datum and design datum did not coincide. Results: The processing and improvement methods of process dimension chain were put forward from three aspects: the selection of concluding link, the reasonable compression of known component link dimension tolerances and the determination of process dimension tolerance according to the actual deviation of other component ring dimensions. Conclusion: By processing and improving the process dimension chain from the above three aspects, the phenomenon of virtual tolerance of process dimension can be effectively avoided.