A method for preparing a crystal of cobalt nitrate from a spent catalyst of Co/SiO 2, the method comprising the following steps: 1) calcining a spent catalyst of Co/SiO 2 in the presence of air for between 3 and 6 hours at a temperature of between 350 and 500°C, cooling to room temperature, and grinding the spent catalyst of Co/SiO 2 to yield a powder; 2) introducing the powder to a fluidized bed reactor, allowing the powder to be reduced for between 8 and 12 hours in the presence of a mixed gas of H 2 and N 2, a volume ratio of H 2 and N 2 in the mixed gas being between 1: 1 and 4: 1; 3) adding a reduced spent catalyst to an excessive dilute nitric acid solution for dissolution, filtering, and collecting a cobalt nitrate solution; 4) adjusting a pH value of the cobalt nitrate solution using an alkali solution to be 1.5, adding a preheated oxalic acid solution having a temperature of between 25 and 80°Cand a pH value of 1.5 to the cobalt nitrate solution in the presence of a water bath of between 25 and 80°C, adjusting a pH value of a resulting solution using a dilute alkali solution to be 1.5, immediately filtering the resulting solution to yield a precipitate of cobalt oxalate, washing the precipitate of cobalt oxalate using deionized water to yield a neutral filtrate, wherein, a mole number of oxalic acid in the oxalic acid solution is controlled to be between 2 and 3 times a mole number of cobalt in the cobalt nitrate solution; 5) drying the precipitate of cobalt oxalate and calcining for between 4 and 8 hours at a temperature of between 550 and 650°C, to yield cobalt oxide; 6) dissolving the cobalt oxide with a dilute nitric acid solution to yield a second cobalt nitrate solution; and 7) evaporating and crystallizing the second cobalt nitrate solution to yield a crystal of Co(NO 3) 2 ·6H 2 O. 1) calcining a spent catalyst of Co/SiO 2 in the presence of air for between 3 and 6 hours at a temperature of between 350 and 500°C, cooling to room temperature, and grinding the spent catalyst of Co/SiO 2 to yield a powder; 2) introducing the powder to a fluidized bed reactor, allowing the powder to be reduced for between 8 and 12 hours in the presence of a mixed gas of H 2 and N 2, a volume ratio of H 2 and N 2 in the mixed gas being between 1: 1 and 4: 1; 3) adding a reduced spent catalyst to an excessive dilute nitric acid solution for dissolution, filtering, and collecting a cobalt nitrate solution; 4) adjusting a pH value of the cobalt nitrate solution using an alkali solution to be 1.5, adding a preheated oxalic acid solution having a temperature of between 25 and 80°Cand a pH value of 1.5 to the cobalt nitrate solution in the presence of a water bath of between 25 and 80°C, adjusting a pH value of a resulting solution using a dilute alkali solution to be 1.5, immediately filtering the resulting solution to yield a precipitate of cobalt oxalate, washing the precipitate of cobalt oxalate using deionized water to yield a neutral filtrate, wherein, a mole number of oxalic acid in the oxalic acid solution is controlled to be between 2 and 3 times a mole number of cobalt in the cobalt nitrate solution; 5) drying the precipitate of cobalt oxalate and calcining for between 4 and 8 hours at a temperature of between 550 and 650°C, to yield cobalt oxide; 6) dissolving the cobalt oxide with a dilute nitric acid solution to yield a second cobalt nitrate solution; and 7) evaporating and crystallizing the second cobalt nitrate solution to yield a crystal of Co(NO 3) 2 ·6H 2 O.