The invention relates to a monitoring system and method for a relation of runoff and rainfall of an underlying surface. The monitoring system comprises a runoff monitoring unit and a rainfall monitoring unit, wherein the runoff monitoring unit is provided with a water insulation fence enclosing a partial surface of the to-be-monitored underlying surface; a gap between the water insulation fence and the to-be-monitored underlying surface is sealed by a sealing material; a runoff collection device is arranged on one side of the water insulation fence, and connected with a runoff water measurement meter; the rainfall monitoring unit is provided with a water collection tank which is identical to a horizontal projected area of the water insulation fence; the water collection tank is provided with a rainfall collector; the rainfall collector is connected with a rainfall water measurement meter; the runoff water measurement meter and the rainfall water measurement meter are connected with a data processing device by a data acquisition device. According to the system and the method, the rainfall capacity and the runoff capacity are measured by a prototype experiment; prototype determination by calculating rainfall and runoff coefficients of the underlying surface is realized; a corresponding relation of the runoff capacity and the rainfall capacity is realized by synchronously monitoring the rainfall capacity; a corresponding relation of the rainfall and runoff capacity and a runoff area is fully considered.