The invention discloses a micron-sized particle stacking compactness testing device and a testing method thereof, and belongs to the technical field of material science and engineering, and the testing device comprises a piston, an outer sleeve, a filter membrane, a stainless steel filter screen, a perforated sheet, a sealing washer, a perforated supporting assembly, a liquid storage assembly and a demolding device. According to the testing device, filtering media with pore diameters and strength from small to large are combined to form the filtering assembly, so that the maximum bearable stress is greatly increased while the rejection rate is ensured, and the application range is enlarged. The invention further discloses the testing method of the device, the close packing state of the particles can be accurately judged, and the packing compactness of the micron-sized particles can be tested. According to the testing device and the testing method, the problems that the influence of particle clusters cannot be completely eliminated, critical condition judgment is subjective and the like in an existing method are solved, accurate testing of the micron-sized particle stacking compactness is achieved, and prediction of the rheological property of a solid-liquid mixture, optimization of the proportion and even guarantee of the final quality of the product in actual engineering are facilitated.