A theoretical basis guarantees the smooth implementation of ecological protection and restoration projects of mountain-river-forest-field-lake-grassland. This paper discusses the application of ecosystem service theory in mountain-river-forest-field-lake-grassland ecosystem conservation and restoration. A cognitive framework of the relationship between mountain-river-forest-field-lake-grassland and ecosystem services on different spatial and temporal scales was established based on the principle of "pressure-state-response", and the key issues of ecosystem services research in mountain-river-forest-field-lake-grassland system restoration projects were analyzed. The key issues were as follows:(1) spatial-temporal evolution and driving mechanisms of major ecosystem services in mountain-river-forest-field-lake-grassland project areas. (2) Trade-offs and synergy among various ecosystem services in mountain-river-forest-field-lake-grassland project areas. (3) Strategies of ecosystem service integration and ecosystem management in mountain-river-forest-field-lake-grassland project areas. (4) Regulation strategy and scale effect of mountain-river-forest-field-lake-grassland ecosystem rehabilitation projects. (5) Exploration of the value realization mechanism of high-quality ecological products. The results of this study will promote the translation of science to practice, and the application of ecological protection and restoration projects of mountain-river-forest-field-lake-grassland.