The invention relates to a device for measuring the total collected rainfall of a roof rainwater vertical pipe. The device is characterized by comprising a water storage pond, a pressure sensor arranged at the bottom of the water storage pond, a siphon arranged in the water storage pond, an automatic control valve arranged at a water inlet of the water storage pond, and a liquid level meter. Whenthe pressure sensor detects that the water storage pond is full of water, the self-control valve is controlled to be switched off, and water inflow is stopped; when the pressure sensor detects that the water storage pond is empty, the self-control valve is controlled to be switched on, and water inflow is recovered. On the basis that an existing building rainwater drainage system is transformed, the device can accurately measure the process of roof runoff discharged from the rainwater vertical pipe in the one-time rainfall process and therefore can quantify the actual effects of roof greeningtransformation on rainfall runoff reduction so that designers and users can know the reduction effect of roof greening on the total runoff volume more visually and more clearly.