A novel strain sensor based on a weakly coupled fiber structure with three tapered cores is proposed. This structure comprises two thin-core fibers and a tapered three-core fiber, in which the tapered three-core fiber is sandwiched between two thin-core fibers. The strain and temperature characteristics of the sensor were studied using various lengths of the tapered three-core fiber. The results show that the interference spectrum of the sensor is blue-shifted with increasing strain in the range of 0--1600 με. With a three-core fiber length of 7.4 cm and a cone diameter of 41.20 μm, the maximum strain sensitivity is -3.47 pm/με, and the linearity is 0.9876. At the same time, we also investigated the temperature sensing characteristics. In the temperature range of 25--60 ℃, the temperature sensitivity of the sensor is 34.52 pm/℃, and the linearity is 0.9979. The cross-sensitivity is 9.95 με/℃. The proposed sensor has the advantages of low cost, simple structure, and wide measurement range, and has potential applications in fields such as industrial production and construction monitoring.