Raw materials in form of powder and liquid are dosed accurately, and then are sent to neutralization and ageing system to be neutralized and aged. The process stream in form of slurry now goes through a milling-homogenizing pump to be homogenized. After that the slurry is then fed to the spray-drying tower to form hollow beads with bulk density of 0.20 to 0.35 kg/L, granulation size of 0.5 to 1.5 mm. After sieved and conditioned in an airlift system, the powder becomes the base powder of low bulk density. A post-addition unit can be provided according to customer's requirements, where perfume, enzyme, perborate are mixed to the base powder to yield medium- or high-grade detergent powder.