1. A monitoring system for a passenger conveying device comprising: an imaging sensor and/or a depth sensing sensor used for sensing a monitoring area of the passenger conveying device to acquire data frames; and a processing device used for performing data processing on the data frames to monitor whether the monitoring area is abnormal, wherein the processing device is configured to: perform a background acquisition process, wherein the background acquisition process includes acquiring a background model based on data frames sensed when the monitoring area is in a normal state; perform a foreground detection process, wherein the foreground detection process includes comparing data frames sensed in real time with the background model to obtain a foreground object; perform a foreground feature extraction process, wherein the foreground feature extraction process includes: defining an abnormal human body model based on human body non-self-learning modeling and/or human body self-learning modeling; and extracting a corresponding foreground object markup feature from the foreground object; and perform a state judgment process, wherein the state judgment process includes judging whether the foreground object belongs to an abnormal population at least based on the foreground object markup feature.