Based on the survey data of agricultural and pastoral households in 14 counties/districts of Shandong, Shaanxi, Shanxi, Ningxia Hui Autonomous Region, Xinjiang Uygur Autonomous Region, Inner Mongolia Autonomous Region, Heilongjiang, and Hebei province, China from 2019 to 2022, the Life cycle assessment (LCA) method was applied in this study. Energy consumption, land use, water resource consumption, global warming, environmental acidification, and eutrophication of traditional farmland and alfalfa grassland ecosystem were calculated and analyzed to evaluate the differences in resource input and environmental effects between traditional farmland and alfalfa grassland ecosystems in Northern China. In this study, LCA method was applied to the environmental impact analysis of two crop production types to explore the effectiveness of this method in the agricultural environment research and the difference characteristics of resource input and environmental effects of the traditional farmland and alfalfa grassland ecosystem. The results showed that:(1) the comprehensive environmental impact index of traditional farmland ecosystem and alfalfa grassland ecosystem were 0.1569 and 0.1269, respectively. The comprehensive environmental effect of alfalfa grassland ecosystem was 19.09% lower than that of traditional farmland ecosystem, and the degree of environmental friendliness was relatively higher. (2) In the whole study region, the environmental impact of traditional farmland was higher than that of alfalfa grassland, and the environmental impact of traditional farmland was significantly different from that of alfalfa grassland, while the overall fluctuation of the environmental impact was small. In terms of resource consumption, compared with the traditional farmland ecosystem, the energy consumption, land area, and water resource consumption of alfalfa grassland ecosystem decreased by 31.21%, 43.61%, and 63.43%. In terms of environmental impact, compared with the traditional farmland ecosystem, the climate warming potential of alfalfa grassland ecosystem decreased by 43.09%, environmental acidification potential decreased by 50.27%, and eutrophication potential decreased by 46.78%. (3) The spatial scale difference of resource utilization and environmental cost between the traditional farmland and alfalfa grassland ecosystems in northern China was obvious, which was higher in the west than in the east. (4) In addition, the major types of environmental impacts affecting both ecosystems were environmental acidification and eutrophication, which were associated with large amounts of fertilizer production, application, and irrigation power consumption. Therefore, the implementation of formulated fertilization, rational irrigation, and straw returning was the key to reduce the negative effect of ecological environment on the life cycle of traditional farmland and alfalfa grassland ecosystem in northern area, and this study proposed a robust methodology contributed to the sustainable development of grassland ecology in the similar region.