The invention provides a testing device and method for simulating comprehensive performance of an underground well cementing sheath. The testing device for simulating the comprehensive performance ofthe underground well cementing sheath comprises an internal tube with a sealed cavity and an external tube, wherein the two ends of the internal tube are connected with each other by using a connecting plate in a sealed manner; the sealed cavity is filled with a pressure medium; the external tube is arranged outside the internal tube in a sleeving manner; an annular cavity is formed between the external tube and the internal tube; a first external blind plug and a second external blind plug can be correspondingly connected to the two ends of the external tube; and a pressure test joint capableof injecting pressure liquid into the external tube is connected to the first external blind plug, wherein the annular cavity is filled with a well cement sheath. According to the testing device forsimulating the comprehensive performance of the underground well cementing sheath, the cementing property of the cement sheath when cement slurry systems and annular clearances are both different canbe tested; and the cementing performance of oil and gas well cement slurry can be very conveniently and accurately measured.