The invention discloses a variable-frequency detecting system and a method for door opening and closing abnormity. The system comprises a door machine system and an elevator control system; the door machine system is communicated and connected with the elevator control system through an internal communication connecting wire; the door machine system is used for outputting corresponding current through a variable-frequency controller to control opening and closing actions of an elevator landing door or car door; the elevator control system is used for monitoring variable-frequency data of the variable-frequency controller to detect if abnormity in the door opening and closing process is generated; and if so, the abnormity type is judged, the fault code corresponding to the abnormity type is output, the floor information and the position information of the door machine system are stored, and the fault information is sent to development and debugging persons. The system needs no additional hardware to detect such information as the obstacle type and the operation state of the elevator door machine, saves the product cost, reduces the development and debugging costs, and can continuously maintain and upgrade the functions and algorithms in the later stage to satisfy the product development demands.