The invention discloses a method of extracting chitin from a shell. The method comprises the following steps: S1, dividing the shell into two parts, crushing the first part of shell into blocks, the diameters of which are 0.5-3cm and crushing the second part of shell into granules, the grain sizes of which are 0.1-0.7cm; S2, then mixing the first part of shells and the second part of shells, putting the mixture in water for ultrasonic treatment, then adding a citric acid solution, and then performing continuous ultrasonic treatment; and S3, soaking the block shell for 30-40h with a sodium hydroxide solution, wherein the process of soaking the shell with the sodium hydroxide solution is divided into two stages: soaking the shell for 20 minutes in the first stage, wherein the concentration of the used sodium hydroxide solution is 12mol/L, and soaking shell in the rest of time in the second stage, wherein the concentration of the used sodium hydroxide solution is 5mol/L; and then soaking the block shell for 2-4h with a hydrochloric acid solution. According to the method disclosed by the invention, the extraction efficiency of chitin in the shell is increased, and the ash of the chitin which is finally extracted is little.