The budget is the most important financial document of the country, which affects the macro economy factors. On the other h, the budget deficit is very important as one of the components that has various economic causes effects. In all studies, the variables affecting the budget deficit, have been studied separately and without considering the relationship between them the solutions, did not have the necessary comprehensiveness; Therefore, this article seeks to draw a diagram in which the causal relationship between the factors affecting the budget deficit its effects can both be seen systematically. For this purpose, system dynamics is a powerful tool. In this method, using the existing backgrounds in this field the opinion of experts, dynamic hypotheses are interpreted then a causal diagram is drawn, which results in that all the factors affecting the budget deficit and their mutual influence, along with their relationship with the results they produce, can be seen in the diagram. The results showed that taxes, government size, private sector credibility, degree of economic freedom, rule of law investment in human resources, in the form of closed loops, which have reinforcing or balancing effects, affect the budget deficit.