The invention provides a roof greening structure and a greening method. The roof greening structure comprises a main structure and an auxiliary structure, wherein the main structure comprises a water-proof root-blocking layer, a protection layer, a light-weight aggregate pervious concrete water storage layer, a filter layer, a light nutrient soil layer and a vegetation layer which are paved on the main structure in sequence; the auxiliary structure comprises a water drainage opening, an inspection well, a spare drain opening and an overflow opening; the material, namely light-weight aggregate pervious concrete, of the light-weight aggregate pervious concrete water storage layer is prepared from the following materials by weight: 50 to 80 parts of pumice, 15 to 45 parts of cement, 5 to 15 parts of water, 0.5 to 1.5 parts of a special strengthening agent for pervious concrete, and 0.1 to 0.5 part of a water-retaining agent. The void ratio of the light-weight aggregate pervious concrete water storage layer is equal to or larger than 20 %, can store rainwater of concentrated rainfall, and has the functions of flood detention, storage regulation, flood peak reduction, and initial rainwater purification in a regional scale; due to the large void structure, plant roots can penetrate easily, so that the roof greening structure and the greening method are suitable for planting low shrubs and dwarf trees, and have a good root fixing effect.