The invention provides a preparation method of 1,1,1,3,3-pentafluoropropane. In the presence of a liquid fluorination catalyst, 1,1,1,3,3-pentachloropropane is reacted with HF (Hydrogen Fluoride) in a liquid fluorination reactor provided with a low-temperature reaction area (1) and a high-temperature reaction area (2) under the conditions that the molar ratio of HF to 1,1,1,3,3-pentachloropropaneis 6-15:1, the reaction pressure is 1.0-1.5MPa, the reaction temperature of the low-temperature reaction area is 60-90 DEG C, and the reaction temperature of the high-temperature reaction area is 90-140 DEG C, wherein the liquid fluorination catalyst is antimony pentachloride or antimony tetrachloride; 1,1,1,3,3-pentachloropropane is introduced into the low-temperature reaction area (1) of the liquid fluorination reactor, fresh HF is introduced into the high-temperature reaction area (2) of the liquid fluorination reactor so as to enter a reaction system, and circulating HF is introduced intoboth of the low-temperature reaction area (1) and the high-temperature reaction area (2) of the liquid fluorination reactor at the same time. The invention is applied to preparation of HFC-245fa.