The invention discloses an automatic smearing device and method for a corrosion-resistant medium on the inner wall of a production string in an acidic gas well. The automatic smearing device comprises a corrosion-resistant medium injection system, a smearing ball injection system, a smearing control and recycling system and an improved gas production tree; the improved gas production tree adopts a structure that two small four-way flanges and one big four-way flange are connected from top to bottom; the corrosion-resistant medium injection system comprises a corrosion-resistant medium storage tank, an injection pump, an annulus high-pressure source, a flow meter, a pressure gauge, a ground injection pipeline and an underground corrosion-resistant medium injection short section; the smearing ball injection system comprises an intelligent ball, a ball throwing tube and an underground ball retaining short section, wherein the ball throwing tube is connected to the small four-way flanges in the middle of the improved gas production tree; the underground ball retaining short section is connected to the lower part of the production string in series; the smearing control and recycling system comprises an airflow velocity and intelligent ball motion control device, a computer control unit and a separation and recovery device. The automatic smearing device and device disclosed by the invention can obviously improve the spraying operation effect of a corrosion inhibitor and is suitable for long-term use.