The invention discloses an early warning device and an early warning method for pavement rainwater drainage. The early warning device comprises a rainfall detection instrument, a water depth display and a PLC chip machine which are arranged on a marginal strip in the inlet direction of a road, wherein the rainfall detection instrument, the water depth display and the PLC chip machine are electrically connected; the rainfall intensity data measured by the rainfall detection instrument is transferred to the PLC chip machine in real time and is computed by the PLC chip machine, and then, pavementwater depth data is displayed on the water depth display; a drainage device is arranged in an intersection of concave-shaped, valley-shaped or saddle-shaped terrains or in the lowest pit of a concave-shaped vertical curve of a road section; a pressure sensor is arranged in the lowest pit of the pavement; the pressure sensor is flush with the pavement; and the pressure sensor is electrically connected with the PLC chip machine. The early warning device disclosed by the invention can prompt early warning for road water accumulation depth, has a quick drainage function, achieves the purpose of maximum and quick drainage, and meets the road traffic safety.