In the process of converting CAD ( Computer Aided Design) models into STL ( Standard Triangular Language) models in three-dimension ( 3D) printing，there exist such problems as reduction of surface accuracy and loss of structure information． In this paper，a adaptive layered method is proposed based on the contour line of CAD models，with an approach of angle grouping，by referring to a series of planes containing Z axis to cut CAD models with equal turning angle， rotating the intersecting contour lines of cutting planes and model surfaces to XOZ plane around the Z axis，and dividing the contour lines into differential segments，applying trigonometric function relations，probability and statistic method，and weighting to calculate the adaptive layered thickness of each layer for 3D printing STL files． The results of model printing show that the algorithm can reduce conversion error of STL files， improve printing accuracy and printing efficiency of 3D printings．