Examining the spatiotemporal evolution characteristics and the constraint factors of the agricultural modernization development level is helpful for promoting high-quality agricultural development and optimizing the top-level design of related fields. Taking 9 provinces in the Yellow River Basin as examples, this paper analyzed the spatiotemporal evolution characteristics and differences of the agricultural modernization level in the Yellow River Basin by the entropy method, the Theil index, and the constraint degree model. Results show that from the perspective of the entire Yellow River Basin, the level of agricultural modernization is constantly improving but still at a low level. From the perspective of the spatial distribution characteristics of the Yellow River Basin, the development level of agricultural modernization in the 9 provinces is in a non-equilibrium state, with Inner Mongolia, Shandong, Sichuan, Qinghai, and Henan having higher development levels, and Gansu, Shaanxi, Shanxi, and Ningxia relatively lower levels, forming a spatial development pattern of “low in the middle, high around”. But the gaps among provinces are gradually narrowing. The constraint factors to the development of agricultural modernization in the Yellow River Basin are mainly concentrated in agricultural basic input and the level of agricultural technology and socialized services. Among them, water resources per unit of farmland, construction of high-standard farmland, per capita farmland, the level of development of agricultural socialized services, and the organization of mechanized social services are common constraint factors in most provinces. Therefore, this paper suggests: 1) to optimize the spatial layout of agricultural modernization; 2) to manage agricultural production scientifically to improve water resource utilization efficiency; 3) to strengthen high-standard farmland construction; 4) to promote agricultural scale management; 5) to improve agricultural socialized service system under policy guidance; and 6) to strengthen technological innovation and improve the level of socialized services for agricultural mechanization.