The optical temperature sensor utilizing the fluorescence intensity ratio (FIR) has garnered significant attention in the past few years due to its rapid response, robust anti-interference capability, remote control feature, and other advantages. In this study, the high-temperature solid-phase approach was used to fabricate a variety of double perovskite-structured La2MgSnO6: Bi3+, Sm3+ (LMS: Bi3+, Sm3+) phosphors. The Rietveld refinement data of XRD and the Gaussian fitting of the emission peak of LMS: 0.02Bi3+ phosphor indicated Bi3+ occupies three lattice sites. The calculation and analysis of average lifetime and energy transfer efficiency substantiated the presence of energy transfer from Bi3+ to Sm3+, with a transfer efficiency of up to 59.07%. The emission intensity of LMS: 0.02Bi3+, 0.05Sm3+ at 403 K maintains 50.2% at the condition of room temperature. The FIR fitting and calculation demonstrated that LMS: 0.02Bi3+, 0.05Sm3+ phosphor possessed good optical temperature sensitivity, with a maximum absolute sensitivity Sa-max of 0.0055 K−1 and a maximum relative sensitivity Sr-max of 0.88% K−1, demonstrating its valuable potential applications for optical temperature sensors.