Abstract: The long-term implementation of the financial budget system for social insurance agency fees in China is difficult to adapt to the inherent requirements of the actuarial system for the accuracy of cost accounting and the stability of funds, which restricts the digital transformation and integrated coordinated development of the national social insurance agency, and does not meet the reform requirements of the actuarial system of social insurance. Based on the construction practice of the financial insurance project, this paper systematically analyzes the realistic predicaments of the financial budget system in the perspective of the sustainability of handling investment, the release of scale effect and international comparison. On this basis, with the actuarial balance theory as the core support, this study demonstrates the advantages of the cost budgeting system by internalizing the handling expenses into the legal cost of the fund and establishing a scientific accounting system; on the other hand, it explains the inevitability of constructing the social insurance actuarial system. The research shows that promoting the transformation of social insurance agency investment system to a cost budgeting system is the fundamental path to break through the bottleneck of social insurance agency investment and ensure its digital transformation. The transformation is not only the concrete embodiment of the implementation of "improving the actuarial system of social insurance" at the Fourth Plenary Session of the 20th CPC Central Committee, but also the main contents of social security system reform in the 15th Five-Year Plan period.