The invention provides a preparation method of 1,1,1,3,3-pentafluorobutane, which comprises the following step of reacting 1,1,1,3,3,-pentachlorobutane with HF (Hydrogen Fluoride) in a liquid-phase fluorination reactor provided with a low-temperature reaction zone (1) and a high-temperature reaction zone (2) in the presence of a liquid-phase fluorination catalyst. The reaction conditions are as follows: the mol ratio of the HF to the 1,1,1,3,3,-pentachlorobutane is 6-15:1, the reaction pressure is 1.0-1.5 MPa, the reaction temperature of the low-temperature reaction zone is 60-90 DEG C, the reaction temperature of the high-temperature reaction zone is 90-140 DEG C, wherein the liquid-phase fluorination catalyst is antimony pentachloride or tin tetrachloride; the 1,1,1,3,3,-pentachlorobutane enters the low-temperature reaction zone (1) of the liquid-phase fluorination reactor; the fresh HF enters the high-temperature reaction zone (2) of the liquid-phase fluorination reactor and then enters a reaction system; and the circulating HF enters the low temperature reaction zone (1) and the high-temperature reaction zone (2) of the liquid-phase fluorination reactor simultaneously. The invention is used for the preparation of the 1,1,1,3,3-pentafluorobutane.