The invention discloses a testing device and method for simulating the flowback in the shut-in period of fractured wells, comprising a fracturing fluid tank, a flowback fracturing fluid tank, a hydraulic fracturing pump, a flowback fluid collection beaker, and a shut-in flowback testing system; the shut-in flowback testing system is composed of a box body and an acoustic wave testing device, a temperature control device, a fracturing string and a square rock block arranged in the box body; a stress loading plate and a hydraulic block are successively arranged between the four surfaces of the square rock block and the inner walls of the box body; the acoustic wave testing device consists of an acoustic wave transmitting probe and an acoustic wave receiving probe; the four outer surfaces of the box body are provided with hydraulic tanks.