The invention discloses a device for measuring moisture content of three phases of oil, gas and water of a ground wellhead. The device comprises a casing, an exhaust pipe, a measurement pipe and a control system, wherein the side wall of the casing is communicated with an inlet pipe, the measurement pipe is arranged in the casing, a closed annular cavity is formed between the measurement pipe andthe casing and communicated with the exhaust pipe, and an exhaust valve exhausts gas in the annular cavity; the measurement pipe comprises a mixed phase measurement pipe and a whole water measurementpipe, the whole water measurement pipe is provided with a water inlet hole and a first conductivity sensor, and the control system collects output voltage of the first conductivity sensor to obtain first voltage; the mixed phase measurement pipe is provided with a liquid inlet hole and a second conductivity sensor, and the control system collects output voltage of the second conductivity sensor toobtain second voltage; the control system determines the moisture content of a to-be-measured fluid according to the first voltage and the second voltage. The device can measure the moisture contentof the ground wellhead in real time and is simple in structure, low in cost and small in size.