The flat roofs were used for the old buildings,whose indoor thermal environments were very poor due to large volume of energy consumption and leakage of roofs.The reforming plan of flat_roof_to_slope_roof is put forward in order to save energy,solve the problem of leakage,raise living amenity and beautify cities.The construction techniques of this sort of projects were introduced by case histories,and the energy_saving economic benefit of flat_roof_to_slope_roof is also analyzed.