Financial security is the foundation and guarantee of economic security. The reform of the county under the direct management of the Ministry of Finance is an important barrier to construct county-level fiscal security defenses and an important way to explain the financial security of local governments in China. This paper selects Shanxi Province as a typical case, and takes the direct management of the Ministry of Finance as a quasi-natural experiment to examine the policy effect of fiscal decentralization reform on the balance of fiscal revenue and expenditure at the county level. Adopting a combination of propensity score matching and double difference method (PSM+DID) to identify the causal effect of reforms on county-level fiscal revenue and expenditure ratios, and from three perspectives: social resource integration capability mechanism, internal motivation mechanism and external pressure mechanism The internal mechanism of the causal disposition effect is verified. Research has shown that the reform of the counties directly under the direct management of the Ministry of Finance has significantly increased the fiscal revenue and expenditure ratio of the pilot counties by 6 percent, and the policy effects are lagging and sustainable. This conclusion is still valid after changing the measurement method, replacing the sample, and testing the placebo. This article provides an explanation path for understanding local fiscal security from the perspective of fiscal system reform, and provides a theoretical basis for continuing to advance and deepen the reform of counties under the direct management of the fiscal province.