The invention discloses a soil moisture sensor based on an RC (Resonance Circuit) resonance principle, which comprises a self-resonance inductor and an osmosis membrane. The sensor measures a moisture content in soil by using stray capacitance change of a resonance circuit caused by dielectric constant change of the osmosis membrane. The sensor solves the problems of the existing soil moisture content detection equipment in debris flow monitoring and is specially used for the monitoring and early warning of a debris flow. The sensor extracts soil moisture with the osmosis membrane, can obtain a dielectric constant of the osmosis membrane by detecting RC resonance frequency of the inductor via the circuit, and finally obtains the moisture content in the soil.