Abstract: There is important theoretical and practical significance to study the change of coastline. Currently, in order to complete coastline analysis work, it is short of the automatic and accurate algorithms which can construct and mark all the changed areas including erosional and increasing types between two period coastlines. We consider that all the coastlines distributed in the two periods have directions which are determined by the order of the points on the coastline, and don't intersect with themselves. Coastlines can only intersect with the ones in the other period which have different direction and will be cut into some polylines. By summarizing the distribution rules of polylines on the boundary of the changed area with different type, we get a number of conclusions and put forward the algorithm which can construct changed areas between two period coastlines automatically and mark them with different type exactly. The coastline polylines constituting edges of changed area always arise a period next to another which has the same start or end point. Removing all the polylines constituting a changed area, it will not affect the others. According to the rules, we can extract the polylines on a changed area's edge and build the area using the points. Judge the type of the changed area by examining the direction of earlier coastline which can be divided into clockwise or anticlockwise. We complete the programming of the function module. Several experiments are designed and implemented with some real coastline data collected in the actual projects. To test the algorithm is reliable or not, we compare the boundary and type of the changed area generated automatically by the algorithms with fact. And the results show that the method is effective and feasible. By our method, the erosional areas which changed into water from land and the accretion areas which changed into land from water can be constructed exactly and automatically according to two period coastlines. This provides assistances for spatial and temporal analysis of coastline change.