In response to the need for real-time temperature measurement inside the motor, a nested tube fiber grating sensor was designed. It uses a ceramic substrate as the inner structure, a 304 stainless steel tube as the outer protection, and a pure silica fiber grating written by a femtosecond laser as the temperature sensing element. An optical fiber sensing test system of constant temperature oil bath was built, and the temperature characteristics of the sensor were studied by the method of cycling temperature rise and fall. Test results show that the sensor presents good temperature measurement performance, and the linear correlation coefficient of the temperature rise and fall experiment is 0.9999. In the long-term cycle experiment, the wavelength repeatability reaches ±2pm, and the hysteresis error is less than 5pm. It has high reliability and can be used for temperature monitoring inside the motor.