This paper introduces a kind of single mode-multimode-single mode intrinsic Fabry-Perot interferometer (SMS-IFPI). We optimize the sensing length of the graded-index multimode fiber (GI-MMF), insertion loss, and multiplexing capability through the analysis and simulation of the optical field distribution in the GI-MMF. Experiment proves that the insertion loss of a single sensing unit from the optimized cascade SMS-IFPI may be decreased from 4 dB to 0.83 dB. Testing the response characteristics of SMS-IFPI temperature sensors in the distribution under the condition of different temperature, the experimental results show that each SMS-IFPI can have better sensitivity and linearity.