Abstract: The backflow from urban underground drainage pipe network has great influences on floods draining. To simulate the impact of floods backflow on waterlogging drainage, a double-layer drainage structure including streets and underground pipelines is established. In case of the water exchange between surface and underground drainage systems via virtual connection passage, the calculation formula of pipeline discharge capacity is deduced, which can stimulate the backflow, and a completed numerical calculation model for urban rain flood drainage is established. Classic case verification shows that: the model can simulate the waterlogging on the street and the backflow processes, and the calculation results are reasonable in flooding situations and are more close to the reality compared with traditional simulation methods.