A video file processing method for making and outputting an event film according to video files is provided. The method includes: acquiring at least one video file and detecting a plurality of events and an occurring time of each event in the video file; extracting a plurality of event clips including the events from the video file according to the occurring time; obtaining an event film by assembling a plurality of concerned clips in the event clips according to a merging rule; and outputting the event film. In addition, a video file processing device and a monitoring system using the method are also provided.