The main objective of this study is to present a model for assessing the maturity of internal financial oversight of the public sector budget in the country's executive agencies using the design science research method. In this study, a mixed research method was used. The statistical population of the study included 131 public sector experts, including accountants and general managers of financial affairs and deputy accountants of the country's executive agencies, managers, deputies and heads of departments and experts in the field of financial oversight in the Ministry of Economic Affairs and Finance, the National Planning and Budget Organization, as well as auditors of the Audit Organization, who were selected using the snowball method. In the qualitative part, in order to collect data, the results and findings of previous researchers were first analyzed using a meta-synthesis approach, and the initial conceptual model of internal financial oversight of the country's public sector budget was identified. In the quantitative part, through the design of a researcher-made questionnaire, the opinions of experts who are responsible for internal oversight of the country's public sector budget were collected and the research data were analyzed using the structural equation modeling technique with the help of SMARTPLS software and the relevant model was finalized. Finally, the maturity assessment model for financial oversight of the country's public sector budget was designed with an internal oversight approach based on the logic of the Capability Maturity Integrated Model (CMMI) and 73 indicators were developed to assess the maturity levels of financial oversight of the public sector budget. The presented model was studied in an executive agency of the country. The results indicate that the relevant agency is at level 3 maturity.