The invention relates to a lysimeter for automatically monitoring a conversion process of soil water and underground water in a paddy rice. The lysimeter comprises a lysimeter area, an observation pond, an air pressure adjusting pipe, a pervious pipe, a valve, a filter, a positioning water tank, a positioning water tank guide rail, a valve, a skip bucket water meter, a water supplementing box, a water supplementing opening, a ventilation pipe and a water level gauge, which belongs to the field of agriculture science, and particularly relates to a device for acquiring conversion amount of soil water and underground water in the field. The lysimeter has the beneficial effects that (1) the lysimeter which can precisely measure the conversion amount of the soil water and underground water in the paddy field is provided; (2) the lysimeter can automatically supplement and discharge the water; (3) the underwater supplement amount measurement precision of the lysimeter can reach 0.07 mm; (4) the water leakage amount measurement precision of the lysimeter can reach 0.01 mm; and (5) the water supplementing and water discharging variation following time of the lysimeter is smaller than 5 minutes.