The invention discloses a soilless culture method of drought-resistant flowers in an open-air roof. The method comprises the following steps of: manufacturing ridges with height of 12-20cm at intervals in the open-air roof by sawdust cement paste; filling a soilless culture matrix composed of 30-50% of residue after mushroom plantation, 30-50% of sawdust, 5-10% of perlite, 3-5% of plant ash and 3-5% of calcium-magnesium-phosphate fertilizer to the ridges; cultivating drought-resistant flower seedlings in the soilless culture matrix; spraying water and killing insects in a wet condition; using 0.1-0.3% of urea and 0.3-0.6% of composite fertilizer water solution to pour the seedlings during the growth period. Plants with strong drought resistance, such as portulaca grandiflora, maguey, ball cactus, cactus, calendula officinalis and sago cycas, are applied to form a roof garden, which remains green, has flowers throughout the year, is colorful and has various shapes, thus saving labor, preventing heatstroke and protecting the roof surface as well as improving ornamental value, economic value, and greening economic, ecological and social effects.