Abstract: Improving grain production is the key to ensuring food security in China, and agricultural machinery input and financial support for agriculture, as the technical support and policy guarantee of agricultural modernization, are the essential ways to improve grain output. Based on the panel data of 33 cities in Guangdong and Guangxi from 2007 to 2019, the spatial Dubin model was used to analyze whether there was a spatial spillover effect of agricultural machinery input on grain output. The panel threshold model was used to analyze whether there was a non-linear impact of financial support for agriculture on grain output. The results showed that: first, there was a significant positive spatial agglomeration effect in agricultural machinery input, and the agricultural machinery input levels in Guangdong and Guangxi were low in the east and high in the west. Second, the impact of financial support for agriculture on grain output was non-linear. With the improvement of agricultural machinery input, the role of financial support for agriculture on the increase of grain production was in an "inverted U-shaped" situation. Therefore, the following policy recommendations are put forward: first, Guangdong and Guangxi should optimize the industrial structure of agricultural machinery and equipment. Second, the promotion of agricultural machinery should be combined with the industrial advantages of each city. The third is that both Guangdong and Guangxi should increase grain production subsidies and agricultural machinery subsidies to increase farmers’ income and grain production.