Water in soil is vital for plants. Plants utilize the water absorbed from the soil for transpiration and producing necessary carbohydrates for plant growth. Soil moisture measurement is important to agriculture and gardening. Numerous soil moisture testing methods were researched and developed. These methods can approximately be categorized as dielectric-based methods and non-dielectric methods. The dielectric-based methods are widely used due to their non-destruction, efficiency and accuracy. The proposed method belongs to the dielectric-based methods. Since the dielectric permittivity of water is far larger than that of dehydrated soil, the permittivity of the mixture of water and soil is mainly determined by the quantities of water in soil. Moreover, the permittivity of dielectric material affects propagation speed of electromagnetic waves. The velocity of electromagnetic wave will be decreased when it travels in dielectric material with large permittivity value. The decrease of velocity of electromagnetic waves is highly related to the soil moisture. The proposed sensor provides a method for measuring the deceases of electromagnetic speed. The sensor consists of eight parts, which are a HF oscillator, two high-speed comparators, two LVDS transmitters, a LVDS differential transmission line for test, a LVDS differential transmission line for reference, two LVDS receivers, a phase detection circuit, and a mean-responding power detector. HF oscillator gives a signal to the inputs of two high-speed comparators simultaneously. High-speed comparators convert the HF signal into two TTL logic level signals. These TLL logic level signals are sent to drive two LVDS transmitters, and convert the TTL signal to LVDS differential signal. LVDS signals are leaded into two differential transmission lines. One is for soil moisture test, the other is for reference. The test transmission line is fully contacted with the soil when it is used in applications. The reference transmission line is waterproofed, which provides a reference signal channel for phase detection. Water in soil decreases the speed of electromagnetic wave guided by the test transmission line. LVDS receivers convert the differential signals back into TTL logic level signals. The TTL signals from the LVDS receivers contain the information about decrease of the speed caused by water in soil. Since the speed of electromagnetic wave guided by the test transmission line is slower than that of the electromagnetic wave guided by the reference transmission line, there is a propagation delay between the signals. The phase detector is utilized to extract the delay, which is converted into a voltage signal by the mean-responding power detector. Latosol and yellow earth samples are utilized in the experiments to calibrate the sensor. A gravimetric soil moisture prediction model is built with determination coefficient R2 of 0.9642 and the maximum absolute prediction error is 2.45% when the HF oscillator works at 50MHz.