1. A method for actively detecting and recognizing a placard, the method comprising: moving an image capturing device according to a maneuver rule, wherein the image capturing device continuously captures an image during the movement; determining whether a placard exists in the image by steps of: segmenting the image into a plurality of blocks; obtaining respectively a pixel color of a largest quantity in each of the blocks as a main color corresponding to the block; finding a representative color of each of the blocks; calculating a contrast between respective representative colors of every two neighboring blocks in the blocks; acquiring at least a high contrast zone in the image according to the contrasts; checking a number of edge points in each of the high contrast zones; and selecting a high contrast zone corresponding to the number of edge points greater than an edge point number threshold value to be a placard area, and determining the placard exists in the image; wherein the step of finding the representative color of each of the blocks comprises: obtaining one of the blocks according to a specific order; comparing the main color of the obtained block with the main color of its neighboring block so as to acquire a color difference therebetween; making the main color of the obtained block to be the representative color of the obtained block together with the neighboring block when the color difference is less than a preset color difference value; making the main color of the obtained block and the main color of the neighboring block to be the representative color respectively when the color difference is greater than or equal to the preset color difference value; and repeating the foregoing steps until finding the representative color of each of the blocks is completed; identifying a content of the placard by steps of: providing an optical character recognition (OCR) database that consists of a plurality of character groups, wherein each of the character groups respectively corresponds to a topological feature and a shape feature, each of the character groups includes a plurality of preset characters, and each of the preset characters respectively corresponds to a preset spatial transformation feature; obtaining at least a character region in the placard area; comparing each of the character regions with the content of the OCR database so as to obtain the preset character corresponding to each of the character regions by steps of: for each of the character regions, capturing the topological feature of the character region, capturing the shape feature of the character region, calculating a spatial transformation feature of the character region, acquiring one of the character groups according to the topological feature and the shape feature corresponding to each of the character groups, calculating respectively a similarity between the spatial transformation feature and the preset spatial transformation feature of each of the preset characters in the obtained character group, and determining a character in the character region to be the preset character corresponding to a highest similarity; and selecting the preset characters respectively corresponding to each of the character regions to be the content of the placard; executing an action according to the content if the placard exists in the image; and repeating the foregoing steps to continuously move the image capturing device according to the maneuver rule and determining whether the placard exists in the image newly captured.