1. A monitoring system for a passenger conveying device, characterized in that the monitoring system comprises: 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, and configured to comprise: a background acquisition module used for acquiring a background model based on data frames sensed when the monitoring area is in a normal state; a foreground detection module used for comparing data frames sensed in real time with the background model to obtain a foreground object; a foreground feature extraction module used for extracting a corresponding foreground object markup feature from the foreground object; and a state judgment module used for judging whether the foreground object belongs to an abnormal population at least based on the foreground object markup feature, and determining that the foreground object belongs to the abnormal population under a situation in which a judgment result is “yes”.