Based on the theory of core diameter mismatch, a kind of fiber-optic sensor composed of a multimode fiber-single mode fiber-multimode fiber (MSM) structure and a fiber Bragg grating (FBG) is proposed for simultaneous measurement of temperature and strain. Due to their different response sensitivities to temperature and strain, the temperature and strain are measured simultaneously. The experimental results show that the temperature sensitivities of the MSM structure and FBG are 0.091 nm/℃ and 0.0102 nm/℃ within the range of 20 ℃~80 ℃, respectively. The strain sensitivities are -0.0013 nm/με and 0.0012 nm/με within the range of 0~650 με, respectively. Using the sensitive matrix, simultaneous measurement of temperature and strain is realized. The maximum measurement errors of temperature and strain are ±0.2 ℃ and ±8.25 με, respectively. With high sensitivity, simple structure, immunity to electromagnetic interferences and a good linearity of the experimental results, the structure has an excellent application prospect in engineering field.