Abstract: Based on the photothermal conversion effect of doped fiber and the principle of Fiber Bragg Grating (FBG) temperature measurement, an all-fiber miniature soil moisture content sensor was developed. The 1480 nm pump laser was used to heat the cobalt-doped fiber embedded in the soil, and the temperature change was measured by the Bragg grating engraved on the cobalt-doped fiber. The in-situ measurement of soil moisture content was realized according to the linear relationship between temperature characteristic value and moisture content in the heating process. In order to verify the feasibility of this method and the accuracy of the sensor, a soil moisture content test platform was built and experiments were carried out. The experiments show that there is a linear relationship between the characteristic value of temperature and soil moisture content; the soil moisture content measured by the all fiber micro soil moisture content sensor is in good agreement with the drying method, and the maximum error is −1.41%, which is obviously better than the conductivity method. This new type of soil moisture content measurement method has the characteristics of miniaturization and low power consumption.