The variation of ambient refractive index and ambient temperature is the main factor affecting the error of optical fiber strain measurement. In this paper, a strain sensor based on the dual-mode fiber (DMF) long period fiber grating (LPFG) is designed. The sensor model structure was designed, and the sensor samples with optimized parameters were produced. The experiment tested the response of the DMF-LPFG sensing structure to the strain, temperature and refractive index in the external environment. Through the Bragg grating (fiber Bragg grating, FBG) written on the single-mode fiber with a UV laser, the cross effect of the ambient temperature is solved. The results of the axial strain experiment show that the axial strain sensitivity of the new structure sensor can reach -5.4 pm/με in the strain range of 0 με~840 με, which is greatly improved compared to the ordinary LPFG. The sensitivity is 58.86 pm/℃ in the temperature range of 25 ℃~80 ℃, showing good linearity. At the same time, the sensor is insensitive to changes in ambient refractive index. The dual-parameter matrix is used to process the strain and temperature sensitivity of the few-mode LPFG and FBG to achieve dual-parameter simultaneous demodulation. The new composite grating structure has good sensing performance and engineering application prospects.