The invention relates to an automatic chemical feeding device and method in a foam-drainage gas recovery well. The automatic chemical feeding device mainly comprises a ground chemical storage tank, a thin pipe valve, a thin pipe, a pressure difference control valve, an oil pipe and a casing. The ground chemical storage tank, the thin pipe valve, the thin pipe and the pressure difference control valve form a chemical feeding channel, the thin pipe and the oil pipe form an annular liquid discharge channel, and the pressure difference control valve mainly comprises a chemical inlet, an upper cavity, a valve spool ball, a conical valve seat, a lower cavity, a spring and a chemical outlet. The upper end of the pressure difference control valve is stressed by chemical gravity (constant in pressure), and the lower end thereof is stressed by return pressure at bottom of the well. The more the accumulated liquid is, the higher the return pressure at the bottom of the well is and the lower the downward pressure difference stressed on the valve spool ball is, so that the spring can overcome the pressure difference to support the valve spool ball upwards and chemicals are begun to be added. After the accumulated liquid is discharged, the pressure difference is larger than elastic force of the spring and presses the valve spool ball to stop chemical feeding. The automatic chemical feeding device can automatically feed and stop feeding chemicals according to level of the accumulated liquid, and the chemicals and the accumulated liquid at the bottom can be guaranteed to be sufficiently mixed.