Abstract: Stable and efficient grain production has been one of the most important challenges in modern agriculture in recent years. Agricultural planting structure has always been in the dynamic adjustment between "non-grain" and "grain-oriented" in China. The incentive effect of various subsidies on grain production has gradually weakened with market-oriented reforms, as well as the ever-increasing production and operation modes. The cost-effective public investment can be expected to serve as a policy tool for the long-term national food security. The construction of high-standard farmland is of great significance to optimize the agricultural planting structure in the various regions. In this study, a quasi-natural experiment was performed on China's high-standard farmland construction policy that was launched in 2010. China's provincial panel data was selected between 2005 and 2017. A continuous differences-in-differences model was constructed with the interaction of the high-standard farmland construction area. The core explanatory variable was utilized as the dummy variable for the duration of policy implementation. An empirical test was then carried out to determine the impact of the high-standard farmland construction policy on the "grain-oriented" evolution of the planting structure. Furthermore, the mechanism variables were introduced, such as the scale of farmland circulation, the increase of total agricultural machinery power, and the level of grain planting specialization. The inherent mechanism and transmission path were then revealed using a two-stage estimation model. The results showed: 1) The baseline regression analysis showed that the high-standard farmland construction policy posed a positive effect on the "grain-oriented" planting structure, which increased the proportion of grain crop planting by an average of 7.5%. Four methods were used for robustness testing, including placebo testing by changing the policy implementation time point, replacing core explanatory variables, considering the lag in policy implementation effects, and considering interfering policies such as grain subsidies, farmland rights confirmation, and agricultural insurance. 2) A joint hypothesis test was conducted on the coefficients before the policy implementation. The baseline model followed the parallel trend test. Furthermore, the dynamic impact analysis showed that there was an enhanced impact each year, as the scope and scale of high-standard farmland construction expanded. The cumulative effects were coupled with the ever-increasing soil fertility and agricultural infrastructure. 3) The impact mechanism analysis showed that the implementation of policies drove the evolution of planting structure towards "grain-oriented" via three pathways: the scale effect brought by land transfer, the improvement of grain planting revenue under mechanized operations, and the realization of external scale economy by upgrading the specialization level of grain planting. Therefore, high-standard farmland construction planning should be strictly implemented to actively adopt farmland engineering measures, such as field leveling, consolidation, and centralized contiguous management, in order to promote the transformation of low- and medium-yielding fields and comprehensive management of agricultural ecology. At the same time, the farmland transfer market can be expected to steadily develop for the high level of agricultural mechanization and agricultural socialization services, with emphasis on the specialization of grain planting. The policy impact of "storing grain on the ground" can also be realized in the construction of high-standard farmland.