Optical fiber sensors based on Multimode Interference (MMI) have received widespread attention due to their distinguished advantages, such as high sensitivity and easy fabrication. In this article, we focus on the progress of five main categories of Single-mode Multimode Single-mode (SMS) structures-based optical MMI fiber sensors. We also summarize their sensing structures, performance, practical application scenarios and measurement sensitivities. Finally, we show the current challenges and prospect future development trend. It is expected that this paper can provide guidance for the research and development of MMI based fiber sensors in the future.