Abstract: Under the carbon emission pattern of “carbon peak and carbon neutral”, agricultural carbon emissions, as one of the main sources of greenhouse gases, have become a key area for emission reduction. High-standard farmland construction is an important measure for promoting green, low-carbon, and high-quality agricultural development. An in-depth investigation of the effects and mechanisms of high-standard farmland construction policies on agricultural carbon emissions can provide an empirical basis for optimizing policy formulation and reducing agricultural carbon emissions. This is of great significance in promoting the development of low-carbon agriculture. Based on the theories of scale economy and division of labor, this study constructed a theoretical model of “high-standard farmland construction-agrochemical input intensity/socialized service-agricultural carbon emission”. Based on panel data from 30 provinces in China from 2007 to 2017, this study analyzed the effect and mechanism of the high-standard farmland construction policy on agricultural carbon emissions using a continuous differences-in-differences approach (DID) and mediation effect model. By measuring the agricultural carbon emissions of each province, this study found that national agricultural carbon emissions showed an inverted U-shaped trend, rising at the beginning, then declining, and peaking in 2015. Regions such as Henan, Shandong, Hebei, Jiangsu, and Anhui are at the forefront of agricultural carbon emissions nationwide, whereas regions such as Beijing, Shanghai, Tianjin, Hebei, and Shandong have higher rates of agricultural carbon emission reduction. The dynamic estimation results showed that the carbon reduction of the high-standard farmland construction policy had a lag effect, and the carbon reduction effect appeared in 2013 and continued to increase gradually. The results of the benchmark regression showed that a high-standard farmland construction policy significantly suppressed agricultural carbon emissions. On average, when all other conditions remained unchanged, implementing a high-standard farmland construction policy reduced agricultural carbon emissions significantly, i.e., by 10.1%. Robustness tests were conducted using the approach of substituting variables and considering the interference of other relevant policies. The results confirmed the positive effect of the high-standard farmland construction policy on reducing agricultural carbon emissions. The results of the mechanism analysis showed that agricultural chemical input intensity and agricultural socialized services played mediating roles in reducing agricultural carbon emissions through the construction of high-standard farmland. The construction of high-standard farmlands suppressed agricultural carbon emissions, mainly by reducing agricultural chemical input intensity and improving agricultural socialized services. Heterogeneity analysis revealed that the carbon reduction effect of the high-standard farmland construction policy mainly occurred in provinces with a high degree of land transfer and in non-food-producing areas. In contrast, it did not have a corresponding carbon reduction effect in provinces with a low degree of land transfer and in food-producing areas. Therefore, the government should strengthen the construction of high-standard farmlands and differentiate the implementation of high-standard farmland construction policies according to local conditions and classifications to give full play to the emission reduction effect. In addition, the government should pay great attention to the role of agricultural chemicalization and socialized agricultural services in carbon reduction effects.