The invention relates to a water level sensor, which comprises a base, a protecting cover arranged at the bottom of the base, a seal disc in seal connection with the inner wall of the base, a magnetic core of which the lower part abuts against the seal disc, a hollow support arranged at the upper part of the base, a capacitor, an inductance coil and a spring arranged in the cavity of the hollow support, wherein the hollow support comprises a hollow first support and a second support which is arranged at the periphery of the first support and fixedly connected with the base; the first support and the second support are connected through a thread; the axial line extension direction of the thread is consistent with the vertical direction; and the inductance coil and the capacitor are both fixed on the first support. The vertical distance between the inductance coil and the magnetic core can be changed by spiral advancing and regressing of the first support and the second support in the invention, thus reducing the resonance frequency of the overall LC (inductance-capacitance) circuit of the sensor and ensuring the initial resonance frequency to be adjustable, further reducing the product rejection rate, reducing production cost and improving production efficiency and economic benefit.