Objective Green roof can reduce and delay the runoff peak of rainstorm through vegetation mitigation and soil media retention. Real time monitoring of rainfall amount, water content of soil media, and runoff are of great significance for scientific research on rainwater retention capacity of roof greening. Method Taking programmable logic controller (PLC) as the core component of hardware control and combining with touch screen, an automatic monitoring system for the rainwater storage effect of roof greening was developed to automatically monitor rainfall events, rainfall amount, air temperature, air humidity and wind speed. Result The measurement results of the real-time monitoring system were consistent with the data of the meteorological department, which indicates that the system can effectively realize the automatic measurement of the overflow amount and the relative humidity of the substrate soil. There were differences in the overflow amount of plant trough and the relative humidity of substrate soil among different vegetations. For example, in a continuous rainfall period during the test, the retention rate of Portulaca grandiflora trough system was 54.75%, the Sphagneticola trilobata system was only 26.63%, and the Sedum lineare system was 38.34%. There were seasonal differences in the water holding capacity of the substrate. The relative humidity of the substrate generally reached more than 70% after the rainfall in August, while the relative humidity of the substrate of Sphagneticola trilobata system was lower than 70% in September, and even lower in October.Conclusion The monitoring system can realize effective, accurate and real-time dynamic monitoring of storage efficiency of outdoor roof greening and ecological environment factors. The rainwater retention capacity, runoff yield characteristics and water retention performance of the matrix soil of the roof plant trough system may be closely related to the meteorological and climatic conditions, rainfall intensity, rainfall amount, plant types, seasonal changes, matrix composition, etc.