At present, the assembly rate of prefabricated buildings in China is calculated manually under relevant specifications according to construction drawings at the design stage. This kind of calculation generates a lot of time and labor costs. Moreover, the same calculation method has different results, and the accuracy is difficult to be guaranteed. In view of this, this paper proposes a calculation method based on BIM.With.Net Formwork4.7 as the framework, C# language was used to carry out secondary development of BIM core software Revit. And the assembly rate calculation method stipulated in the Prefabricated Building Evaluation Standard is incorporated into the plug-in, and the assembly rate calculation of the main structure is realized by establishing the component custom coding system, identifying and obtaining the component data information to calculate the assembly rate of main structure of Precast Concrete (PC). Finally, this paper uses a practical engineering project as an example to validate the feasibility of this method. The research shows that the calculation time and labor cost are greatly reduced and the precision is improved.