Under the conflict between economic returns from grain cultivation and strategic grain functions, it is essential to analyze the impacts of the comparative returns from grain cultivation and the financial support to agriculture on the “non-grain” of farmland in order to put forward strategies for the management of “non-grain” and stabilize the security of domestic grain supply. Based on the national panel data from 2009 to 2021, this paper adopts the dynamic spatial Durbin model and the panel threshold regression model to analyze the situation. Results show that: 1) from 2009 to 2021, China’s “non-grain” farmland has prominent spatial differentiation characteristics, showing a “high-high, low-low” aggregation area bipolar distribution; 2) the comparative income from grain cultivation can inhibit “non-grain” farmland , and has a spatial spillover effect; 3) the inhibition of financial support for agriculture on the “non-grain” farmland has a double threshold characteristics, showing a trend of “first increase and then decrease”. Therefore, to promote the construction of compensation mechanism for the interests of grain, this paper recommends to establish an inter-provincial horizontal interest compensation mechanism in the production and marketing areas, to protect the interests of the main body of grain in the “production end”; to integrate the regional grain comparative income and the reasonable adjustment of the agricultural planting structure; and to improve the supervision system of financial support for agriculture.