BIM top down design is the ideal mode in the design stage. However, there are some problems e.g. data separation between calculation model and drawing model, large workload of input of reinforcement parameters, and inadequate plugins for Revit itself. This paper explores the process of automatic beam reinforcement, which were splitting by span, importing calculation result, calculate reinforcement case of beam, tagging beam reinforcement result, completing additional transverse and stirrup reinforcement, compliance inspection, etc. This method can realize the strong connection between calculation model and drawing model, high degree of automation, and is easy to check and customize the rule reinforcement. The input of beam reinforcement improves BIM data and makes the BIM deliverables more complete, which is more conducive to downstream major.