The invention relates to a method for testing the water content of soil based on a theory of phase detection and a method thereof. The device comprises a high-frequency sinusoidal signal generator, a signal separating circuit, a probe, a circulator, a phase detector, etc. The phase detector is used for receiving a reference signal sent by the signal separating circuit and a reflection signal sentby the circulator, and outputting the phase difference direct voltage signal between the reference signal and the reflection signal to an AD converting circuit; the AD converting circuit is connectedwith the phase detector and is used for converting the direct voltage signal generated by the phase detector into a digital signal to be inputted into a microprocessor; and the microprocessor is usedfor controlling the working frequency of the sinusoidal signal generator, computing the broadcasting time of a test signal on the probe according to the digital signal output by the AD converting circuit, and computing the water content of the soil with a publicly known experimental formula according to the broadcasting time. The device not only can be realized in technology, but also has low cost and is easily popularizedd and used in domestic.