Task DAG is a mean of depicting dependency among tasks of a program. Apart from dependency between tasks, amount of computation and traffic between tasks are also reflected on DAG. All these information together provides a basis for scheduling. There are much research on scheduling base on DAG both domestic and abroad, but research on constructing task DAG based on analysis of correlation among serial program is rare. This paper focuses on analysis of data dependency and control dependency and discuss respectively on sequential, branch and loop structure, propose an algorithm for constructing task DAG of serial programs.