An installing method of a rainwater accumulator for greening structural soil comprises the steps that bottom soil of a land mass of a hard road surface required to be greened is firstly dug away to form pits, then the positions of planting holes are determined, water draining devices are arranged in the pits, and then structural soil and planting soil are laid in a layered mode. The installing method adopts the installing mode that the structural soil and the planting soil are laid in a layered mode, the connecting problem of gaps of the structural soil and the planting soil in the hard road surface is solved, namely the plant planting requirements can be met, the weight bearing requirements of the hard road surface can be also met, timely rainwater permeation can be ensured, a part of rainwater is accumulated in the structural soil, the rest is partially drained to a municipal pipeline in time, impounded surface water is decreased, plant watering times are decreased, and the effect of the rainwater accumulator is played.