To address the issue of temperature cross sensitivity in fiber optic grating sensors, a novel sensing system based on few mode fiber long-period gratings is proposed. This system utilizes the characteristic of few mode optical fibers that can transmit fundamental modes and a small number of higher-order modes, and achieves effective coupling between core modes and cladding modes by adjusting the grating period. The system is composed of two long-period gratings with different periods connected by a coupler, and connected to a broadband light source and spectrometer. Using matrix method to analyze the sensitivity differences of fiber core mode and cladding mode coupling in gratings under different temperature and strain conditions, in order to achieve simultaneous measurement of temperature and strain. The experimental results indicate that within the temperature range of 0 to 100 ℃,the grating wavelength undergoes a red shift, with temperature sensitivities at the troughs Peak₁ and Peak₂ being 0.512 86 nm/℃ and 0.76143 nm/℃, respectively, after coupling. Under a stress range of 0 to 500 με, the grating wavelength experiences a blue shift, with strain sensitivities at the two troughs being -0.138 57 nm/με and -0.082 29 nm/με.