The invention relates to a mud density detection device for a cutter suction dredger and a detection method thereof, which belong to the technical field of dredging engineering. The mud density detection device for the cutter suction dredger is characterized in that the detection device comprises a mud pipe nipple cutoff valve, a sampling bucket and a sampling valve, wherein the sampling bucket is positioned at the lower part of the mud pipe nipple cutoff valve; and the lower side of the sampling bucket is connected with the sampling valve. The mud density detection method for the cutter suction dredger is characterized in that mud sampling density detection is performed by the mud pipe nipple cutoff valve, the sampling bucket and the sampling valve, and the method comprises the steps of: closing the sampling valve, opening the mud pipe nipple cutoff valve, making mud enter the sampling bucket, and closing the mud pipe nipple cutoff valve for mud density detection. The detection device has the advantages of convenient carrying, convenient disassembly and assembly, and no blocking of mud particles; and the detection method has the advantages of simple operation, accurate detection, strong practicability, quick sampling speed, no need of the shutdown of the cutter suction dredger during the detection and the like.