In this paper, a high-sensitivity fiber grating refractive index sensor based on direct femtosecond laser writing is proposed, and the temperature and axial strain characteristics of the sensor are also measured. A long-period grating with a period of 150 μm, a period number of 50, and a gate length of 7.5 mm was inscribed with a femtosecond laser at a wavelength of 800 nm. The sensitivity of the sensor in refractive index measurement experiment is up to 1 605 nm/RIU and a temperature sensitivity of 76.52 nm/℃ in the temperature range of 30 ℃ to 330 ℃. The sensor is stable and insensitive to strain with excellent robustness. This long-period fiber grating based on femtosecond laser direct writing with high refractive index sensitivity and high temperature resistance has a wide application prospect.