According to the real-time crack monitoring demand of metalli c structures during their service, a film sensor is presented to monitor the met al structure crack based on the electric potential method. Firstly, the correspo ndin g finite element model(FEM) of the film sensor is established, and the output c harac teristics of the sensor are analyzed by FEM. The results indicate that the exte nsion direction and the length of crack can be obtained through analyzing the el ectr ic potential differences between monitoring points. And then the film sensor is prepared on the surface of the 2A12 aluminum alloy specimen by ion plating techn ology, which can integrate with metal surface firmly. Finally, crack monitoring experiments are carried out with the film sensor. Experimental results show tha t the structural crack propagation stage can be learned through analyzing the ch ange extent of the adjacent monitoring data of the film sensor.