[Objective] The paper aims to explore the carrying capacity of the green shrub roof and the purification effect on initial roof runoff in order to provide theoretical support for green roof designing and runoff resources utilization.[Methods] A green shrub roof structure was designed according to the carrying capacity of roof and the characteristics of roof runoff. Using vermiculite and perlite as water absorbent, and mixed with red loam and humus at a certain proportion. we made a substrate for roof greening. The roof shrubs were sprayed with simulated rainfall using collected roof runoff, and the dry and wet load and the effect of rainwater removal had been researched.[Results] The saturated water absorption of 1 m³ artificial soil was 0.65 m³, and wet bulk density of the substrate was 1 190 kg/m³. The carrying capacity of green shrub roof under dry and wet conditions was 261 kg/m² and 529 kg/m², respectively. The pollutants SS, COD, TN and TP in initial roof runoff were reduced by 60.1%~71.3%, 50.0%~61.8%, 49.2%~53.2% and 55.6%~67.7%, respectively.[Conclusion] The green shrub roof can meet the carrying capacity requirements of general building. Meanwhile, it has good purification effects on initial roof runoff.