The invention discloses a monitoring device and a monitoring method for detecting a downhole corrosion situation. The monitoring method comprises the following steps of recording data of a monitored hanging piece before entering a well; enabling the monitored hanging piece to descend into the well together with a downhole manometer by fixing the monitored hanging piece on a monitoring device; using the monitored hanging piece as an anode, and enabling the monitored hanging piece to be in contact with a fluid medium in a downhole pipe column to take an electrochemical reaction, so that the monitored hanging piece is subjected to corrosion, scaling and deposition scaling by the fluid medium; salvaging the monitored hanging piece and the monitoring device out of a wellhead together; counting data of the monitored hanging piece entered the well, and comparing the data with the data before entering the well so as to obtain the corrosion situation of a downhole device. According to the monitoring device and the monitoring method for detecting the downhole corrosion situation, disclosed by the invention, monitoring data can be obtained through weighing the monitored hanging piece and carrying out pitting corrosion measurement, the corrosion can be monitored in real time, and corrosion and scaling situations of devices such as the downhole manometer and the pipe column can be really reflected.