Abstract: Abstract：Along with the rapid industrialization and urbanization, China is undergoing increasingly aggravated cultivated land conversion. Consequently, it causes great damage to environment and the worsening of the greenhouse effect. Therefore, this paper puts the cultivated land conversion and carbon budget into a unified framework, and uses the system dynamics method to simulate and measure the carbon budget change caused by the farmland conversion from 1996 to 2020. Using the system dynamics software Vensim, the carbon budget system model based on cultivated land conversion is built, which emphasizes the population subsystem, economic subsystem, cultivated land subsystem and construction land subsystem. Then the dynamic behavior of each system is simulated and the change law is revealed. Firstly, through simulating and comparing the data of the cultivated land and construction land from 1996 to 2020, it is found that the cultivated land conversion is concentrated in the eastern region, and the area of China's cultivated land is decreasing while the construction land is increasing. The simulation results indicate that the area of cultivated land has reduced from 1.293×108 hm2 in 1996 to 1.204×108 hm2 in 2020, and the construction land has increased from 2.407×107 to 3.073×107 hm2. Secondly, in the process of simulating the national carbon budget, the paper mainly selects the 4 variables which are construction land green area, ecological restoration, cultivated land and construction land. And their carbon sink/source correlation coefficients are obtained from previous research. The carbon budget measurement shows that from 1996 to 2020, the carbon sink based on national cultivated land conversion reduces from 7.90×108 to 7.48×108 t, the carbon source increases from 9.34×109 to 1.17×1010 t, and the national carbon budget of the cultivated land conversion generally presents that the carbon expenditure increases from 8.55×109 to 1.10×1010 t. Thirdly, the paper divides China into 4 regions (northeast, east, center and west) and makes regional comparison. The selection and estimation of parameters are similar to the whole country. The regional simulation results show that the carbon expenditure in the east is 3.569×109 t, in the west is 2.065×109 t, in the central is 1.991×109 t, and in the northeast is 8.99×108 t in 1996; and in the year 2020, the carbon expenditure in the east is 5.029×109 t, in the west is 2.261×109 t, and in the central is 2.216×109 t, and in the northeast is 1.084×109 t. And the capacity of carbon sink will be decreasing in 4 regions, the carbon source will be increasing significantly in future, and the carbon budget will present carbon expenditure which is predicted to increase but the increase is not huge. By comparison, the carbon source and carbon budget in east will increase mostly. The carbon sink, carbon source and carbon budget will be stable in northeast. So, the imbalance status of carbon budget in China is mainly caused by the eastern region.