The invention relates to a lane departure warning system and a method based on a monocular camera. The system comprises an image collection module, a processor, a vehicle body interaction module, a violation image storage module, a display, a warning device, a synchronous dynamic random access memory (SDRAM) and a power supply module. The system is used for collecting the information of a road and vehicle bodies in a real-time manner, when the system detects that a vehicle has a tendency to deviate from the lane, the early warning system gives out a warning voice signal and a visual signal to remind a driver to timely correct the deviation of the vehicle, and at the same time, when the system judges that the driver performs a violation operation, the system records violation information in the violation image storage module. The method comprises an express lane filtering algorithm and a candidate lane scoring method for overcoming the defects that the real time performance is poor, the method is liable to be interrupted by environment of multiple lanes, railings on two sides of the lanes, and the like when a Hough algorithm is used for detecting the lines of the lanes; according to the detected parameters of the lane lines and the information of the vehicle bodies, the system can judge whether the vehicle deviates from the lane and whether the driver has the violation operation.