The invention relates to a priori analysis based iterative method for segmenting characters of a licence plate. Method comprises the following steps of: firstly, preprocessing the image of the licence plate, segmenting the preprocessed image of the licence plate by utilizing the vertical projection method, calculating the parameter of each character block, and carrying out preliminary screening according to the parameters; secondly, carrying out binaryzation on the characters processed by preliminary screening by utilizing the threshold value iteration method, calculating the priori knowledgecontained by each character block according to the standard licence plate system, and obtaining the characters processed by secondary screening; and thirdly, calculating the quantity of blocks of thecharacters processed by secondary screening, calculating the width of characters of the licence plate and the mean width of all characters under the condition that set conditions are met, and determining whether ultimate characters can be obtained or not according to the two parameters, or carrying projection again when the set conditions are not met. The method reduces the requirement on positioning accuracy of the licence plate, overcomes the adverse effects due to a complex background, eliminates the common segmentation results of conglutination, breakage and the like of the other methods,and further improves the accuracy of segmentation.