Abstract: China must achieve its goal of fully building modern socialist power by 2050. Agricultural power is the foundation of modern socialist power, and agricultural modernization is necessary for building modern agricultural power. Shandong Province is the first major agricultural province in China, with a total output value of agriculture, forestry, animal husbandry, and fisheries exceeding one trillion Yuan (valued at 1019.06 billion Yuan). Exploring the spatiotemporal evolution and influencing factors of the development level of agricultural modernization in Shandong Province can help accelerate the development process of agricultural modernization, provide a scientific basis for realizing the transformation from a major agricultural province to a modern agricultural province, and provide a reference for development planning in other regions to achieve agricultural modernization. Existing researches neglect the heterogeneity analysis of the internal structure evolution and key constraints of the regional agricultural modernization development level, lack temporal and spatial analysis of the evolution of the agricultural modernization development level, and do not explore the external factors that affect the development level of agricultural modernization. This study used a multi-objective comprehensive measure to evaluate the development level of agricultural modernization in Shandong Province and 16 cities, analyzed the spatiotemporal evolution using exploratory spatial data analysis methods, and introduced an obstacle model and a spatial econometric model to explore internal constraints and external drivers. The results show that: 1) The development level of agricultural modernization in Shandong Province and the scores of production inputs, industry and operation, output benefits, green development, and rural community subsystems showed a fluctuating upward trend from 2010 to 2020, with the rapid development of industry and operation subsystems being prominent. However, the development of production inputs and output benefits subsystems was unstable. The development level of agricultural modernization in various regions and cities had formed differences between “high but unstable” and “low-level traps”. The internal structures of most regions and cities were gradually balanced with the main types of business and social leadership. 2) The spatial manifestation was significant spatial agglomerations. High levels of agricultural modernization development were concentrated in the eastern coastal areas and expand to inland areas, whereas low levels were gradually concentrated in the five cities of South Shandong. There was an abnormal spatial distribution at the junctions of the high- and low-level clusters. 3) The level of electrification restricted the development of agricultural modernization in more than 80% of the prefectures and cities, whereas medical conditions restricted most prefectures and cities with low levels of agricultural modernization. Input-type constraints gradually transformed into industrial- and output-type constraints; urbanization level, science and technology level, education investment, and economic development level all significantly and positively affected the development level of agricultural modernization, and science, technology, and education investment in addition to having significant spatial spillover effects. Therefore, it is necessary to take advantage of local conditions, coordinate internally balanced development, strengthen regional cooperation and exchange, reduce internal constraints, and strengthen external factors driving spillover effects.