Abstract: During the implementation of the combined development mode of the second tertiary method, changing the displacement medium will significantly change the production and water-bearing operation regularity. The recoverable reserves calculated by using the conventional water drive characteristic curve cannot reflect the actual development status of the combination of the second tertiary method. Therefore, according to the analysis and research of water cut change regularities at different stages, the relational expression of water cut change regularity after changing the displacement medium is established, and the ultimate recovery and recoverable reserves of the second tertiary combined block are calculated according to the production principle of fixed liquid. The application results show that the results obtained by this method are reasonable and reliable, and the difficulties caused by the change of displacement medium in the process of combination of the second tertiary method are solved.