With the rapid development of the construction industry, controlling construction costs has become a crucial factor in ensuring the smooth progress of projects. The traditional cost management model faces challenges such as information silos, untimely data updates, and inaccurate cost control, which can easily lead to project cost overruns and schedule delays. This paper takes a large prefabricated building as the research object, and based on BIM technology modeling optimization and AHP entropy weight method, analyzes the weight of functional indicators of factors affecting building costs. The measured calculation results show that the material, labor, and efficiency aspects during the construction process have been optimized and controlled, with a cost reduction of 9.6%, achieving precise cost control for auxiliary building construction.