Food security is a critical national strategy and an essential foundation for the stable economic and social development of China. Curbing the excessive expansion of non-grain production is a key strategy and presents challenges to the preservation of arable land and the progress of agricultural development. This study detects the conversion of cropland to non-grain aquaculture using remote sensing imagery, applies the spatial autocorrelation technique to analyze the county-level spatial distribution of non-grain aquaculture across China, and explores the spatiotemporal dynamics of non-grain conversion across various scopes, such as agricultural regions, grain-producing zones, and provincial administrative divisions. Subsequently, the Geographically and Temporally Weighted Regression (GTWR) model is utilized to explore the factors influencing non-grain conversions. The study reveals several findings: ① Over the past 25 years, there has been a nationwide conversion of arable land to aquaculture, showing an initial surge followed by a subsequent decline in the general trend. ② Non-grain conversions are predominantly concentrated in 110°E-120°E longitudes and 20°N-30°N latitudes, with significant clusters observed in Northeast China, the middle and lower reaches of the Yangtze River, and the Pearl River Delta. Notably, the middle and lower reaches of the Yangtze River have the highest number of counties with such clusters. ③ From the perspective of agricultural zones, the non-grain transformation to aquaculture in the plain and hilly areas of the middle and lower reaches of the Yangtze River has remained at a high level for the past 20 years. In the main grain-producing areas, non-grain conversion has been substantial and continuously grown over the past 25 years, reaching a proportion of 80.2% from 2005 to 2020. The most significant conversions have occurred in Jiangsu, Hubei, Guangdong, and Anhui Provinces. ④ The non-grain transformation to aquaculture is affected by four main factors: natural elements, socioeconomic conditions, the state of cultivated land and agricultural development, and freshwater aquaculture conditions. Elevation has a negative effect, while population growth and the fiscal revenue-expenditure ratio generally promote non-grain conversion in most counties. The area of cultivated land correlates positively with this conversion. Additionally, the non-agricultural use of cultivated land positively impacts most counties in Eastern and Western China, and an increase in freshwater aquaculture production is likely to encourage the non-grain conversion of cropland.