Generalization is the most important among the procedures of map compilation. Its function mainly to maintain the legibility of maps and made the subject prominently by simplifying, seleciting and canceling the complicated map information. Generalization has three main procedures: EVALUATION, SELECTION and EMPHASIS. Evaluation is the first procedure, that includes analyzing the reliability of map information, thus be the criteria of selection in accordance with the map subject. Selection of information includes number of selecting and determining the criteria, these two items should be consistent with each other. Emphasis is the final and most important procedure during the operations of generalization, for it controls the type of data display, and maintains the balance and beauty of maps. Previously, the operations of map generalization down by compilers, the disparity and subjectivity are prominent for the absence of precise rules and precepts; furthermore, this has preventing the principles of automation of map-making. This paper discussed the principles of generalization and its procedures; the aim is to promote the objectivity during operation, let the map display geographical environment faithfully and consistently, besides to match the development and application of computerized map-making.