Water and soil loss affects the carbon and nitrogen cycles of terrestrial ecosystems, forest vegetation ecosystem products and services, and ultimately the quality of life and sustainable development of the public. China has the most serious soil erosion in the world, notably on the Loess Plateau. After years of efforts, soil and water conservation in China has developed rapidly, the surface runoff and soil loss in soil and water loss areas have decreased rapidly, and people’s living standards have gradually improved. With these improved living standards, people have higher requirements for soil and water conservation. However, soil and water conservation lacks scientific theoretical guidance. In this paper, through comprehensive analysis of relevant literature, a new theory of soil and water conservation is proposed. The results shows that soil and water losses refer to the process of transferring soil and water resources from one place to another, and the consequences of these losses can be divided into positive and negative effects. Soil and water conservation is not only the use of some methods or measures to reduce soil erosion to soil allowable loss requirements, but also to make efficient use of soil and water resources. The construction standard of soil and water conservation measures must be based on the allowable amount of soil erosion and be applied using spatially optimal allocation, and the work of soil and water conservation should ensure regional ecological security and realize the sustainable development of soil and water conservation.