A method for detecting media at different depths, comprising the following steps of: S01: providing a sensor, the sensor consisting of a group of PCB copper foils, comprising: a big copper foil which is used as a big polar plate and two small copper foils which are used as small polar plates; S02: providing a depth detection signal Ftest, and applying the depth detection signal Ftest to the big and small polar plates of the sensor in the S01 to form an electromagnetic field between the big polar plate and the small polar plates; S03: providing a self-calibration signal Fcal acting on the big polar plate, to adjust a phase difference between the big polar plate and the small polar plates; and S04: providing a signal shaping and phase comparison circuit to perform shaping and phase comparison on signals output from the big and small polar plates driven by the depth detection signal Ftest to generate phase-comparison signals, and processing signals output after filtering the phase-compared signals to judge the condition of a medium at the current detection depth, wherein the signal shaping and phase comparison circuit comprises a first NAND gate and a second NAND gate; a first input terminal of the first NAND gate is connected to the big polar plate, a second input terminal thereof is connected to an output terminal of the second NAND gate and serves as an output terminal of the signal shaping and phase comparison circuit, and an output terminal thereof is connected to a second input terminal of the second NAND gate; and, a first input terminal of the second NAND gate is connected to the small polar plates; before acting on the big polar plate, the self-calibration signal Fcal is to be filtered by a first second-order RC filter circuit, the first second-order RC filter circuit comprising a third resistor, a first capacitor, a fourth resistor and a second capacitor; the self-calibration signal Fcal is a PWM signal with a fixed frequency and a variable duty ratio generated by a PWM calibration circuit, and the phase difference between the big polar plate and the small polar plates can be changed by changing the duty ratio of the PWM signal; and in S04, the phase-compared signals are filtered by a second second-order RC filter circuit, the second second-order RC filter circuit comprising a fifth resistor, a third capacitor, a sixth resistor and a fourth capacitor. S01: providing a sensor, the sensor consisting of a group of PCB copper foils, comprising: a big copper foil which is used as a big polar plate and two small copper foils which are used as small polar plates; S02: providing a depth detection signal Ftest, and applying the depth detection signal Ftest to the big and small polar plates of the sensor in the S01 to form an electromagnetic field between the big polar plate and the small polar plates; S03: providing a self-calibration signal Fcal acting on the big polar plate, to adjust a phase difference between the big polar plate and the small polar plates; and S04: providing a signal shaping and phase comparison circuit to perform shaping and phase comparison on signals output from the big and small polar plates driven by the depth detection signal Ftest to generate phase-comparison signals, and processing signals output after filtering the phase-compared signals to judge the condition of a medium at the current detection depth, wherein the signal shaping and phase comparison circuit comprises a first NAND gate and a second NAND gate; a first input terminal of the first NAND gate is connected to the big polar plate, a second input terminal thereof is connected to an output terminal of the second NAND gate and serves as an output terminal of the signal shaping and phase comparison circuit, and an output terminal thereof is connected to a second input terminal of the second NAND gate; and, a first input terminal of the second NAND gate is connected to the small polar plates; before acting on the big polar plate, the self-calibration signal Fcal is to be filtered by a first second-order RC filter circuit, the first second-order RC filter circuit comprising a third resistor, a first capacitor, a fourth resistor and a second capacitor; the self-calibration signal Fcal is a PWM signal with a fixed frequency and a variable duty ratio generated by a PWM calibration circuit, and the phase difference between the big polar plate and the small polar plates can be changed by changing the duty ratio of the PWM signal; and in S04, the phase-compared signals are filtered by a second second-order RC filter circuit, the second second-order RC filter circuit comprising a fifth resistor, a third capacitor, a sixth resistor and a fourth capacitor. a first input terminal of the second NAND gate is connected to the small polar plates; before acting on the big polar plate, the self-calibration signal Fcal is to be filtered by a first second-order RC filter circuit, the first second-order RC filter circuit comprising a third resistor, a first capacitor, a fourth resistor and a second capacitor; the self-calibration signal Fcal is a PWM signal with a fixed frequency and a variable duty ratio generated by a PWM calibration circuit, and the phase difference between the big polar plate and the small polar plates can be changed by changing the duty ratio of the PWM signal; and in S04, the phase-compared signals are filtered by a second second-order RC filter circuit, the second second-order RC filter circuit comprising a fifth resistor, a third capacitor, a sixth resistor and a fourth capacitor.