A method for generating and updating position distribution graph comprises: generating a position distribution graph according to a circuit bitmap and an exposure pattern, performing an exposure simulation according to the position distribution graph to generate an exposure result graph, comparing the circuit bitmap with the exposure result graph to generate a plurality of error distribution candidate graphs, selecting one of the error distribution candidate graphs to serve as an error distribution graph, and performing a zero-one integer programming to update the position distribution graph according to the circuit bitmap and the error distribution graph, wherein the updated position distribution graph is associated with the error distribution graph.