1. A method for detecting a vehicle sunvisor's state, comprising: obtaining a detected image; conducting a gray-scale preprocessing on the detected image, to obtain a gray-level image; conducting a plurality of main connected regions extraction on the gray-level image, and calculating a geometric feature and a rectangle similarity of each main connected region; conducting a plurality of horizontal long edges extractions on the gray-level image, and conducting a feature matching operation between the horizontal long edges and the main connected regions, to obtain a plurality of region-edge matching relationships; and determining the sunvisor's state, based on the region-edge matching relationship, the geometric feature of main connected regions and the rectangle similarity of main connected regions.