For the problem of efficiency for Minimum Test Case Set on MC/DC, based on MC / DC criteria and the complex relationships of combination of conditions and decision, the paper turnned a original boolean expression into a syntax tree, then turned the syntax tree into a graph. According to the effect on nodes of the graph to the decision, the paper proposed the graph-based fast algorithm to generate the smallest set of test cases. Finally, the paper verified and compared the algorithm with others, the results show that the algorithm has practicality and superiority.