In the era of media convergence, many excellent IP films have appeared in China's film market. However, focusing only on short-term fan traffic does not bring long-lasting brand benefits for excellent IP. At present, most of China's IP resources blindly copy the successful cases of foreign transmedia storytelling, and do not give full play to their own advantages. Confusing the basic theoretical boundaries and ignoring the focus of theoretical practice have become the main problems facing China's IP film industry. Therefore, China's IP films need a clearer direction for guiding theoretical practice. This paper delves into the theoretical connotation of transmedia storytelling and sorts out the basic framework. With the help of literature research method and comparative analysis method, it compared the characteristics of One Source Multi Use (OSMU) and transmedia storytelling theory, and clarified the boundaries between similar theories. Combined with the situation of Chinese IP films and the structure of transmedia storytelling, the paper put forward the characteristics of three aspects, namely, media form, story world and narrative content, and provides a direction for the theoretical practice. On this basis, this paper took the more successful "Detective Chinatown" IP (hereinafter referred to as "Tang Tan" IP) in China as an example, analyzed the three aspects mentioned above, and verified the feasibility of these three characteristics in Chinese IP films. The results provide a clear direction for the theoretical guidance of transmedia storytelling as well as theoretical and practical ideas for Chinese IP films.