The utility model provides a manhole cover opening state alarm device based on signal intensity change, which comprises a shell fixed on the lower surface of a manhole cover. Wherein an inclination angle detector, a signal transmission module, a signal detection module, a processing module, a positioning module and an analysis and comparison module are packaged in the shell; the output end of theinclination angle detector and the output end of the signal transmission module are connected with the input end of the signal detection module; the output end of the signal detection module is connected with the input end of the processing module; the output end of the processing module is connected with the input end of the user side; the output end of the positioning module is connected with the input end of the analysis and comparison module, and the output end of the analysis and comparison module is connected with the input end of the user side. The manhole cover has the advantages thatwhether the manhole cover is in an open state or not can be accurately judged according to the calculated signal difference value on the premise that the signal intensity difference is large after themanhole cover is closed and opened, the real-time position of the manhole cover can be monitored in cooperation with the positioning module, and a user side can conveniently find the manhole cover intime.