The invention discloses a preparation method for producing chitosan based on shrimp shells. The method comprises the following steps that the shrimp shells are collected, cleaned through tap water, then dried and smashed to obtain shrimp shell powder, the shrimp shell powder is placed in purified water, an acid solution is dropwise added in the stirring process until no bubble is generated, filtering is conducted, solid powder is collected, water washing is conducted until the solution is neutral, and the shrimp shell powder after minerals are removed is obtained and placed in an alkaline solution for heating. The acid solution is dropwise added in the process of stirring the shrimp shell powder/purified water mixed solution, so that the acid solution quickly reacts with the minerals afterentering the mixed solution, the reaction time can be accurately judged, and the situation is avoided that since the mixed solution system is always in high acidity, chitosan is degraded under the acid catalytic effect; it is guaranteed that the relative molecular weight distribution of the chitosan product is in a narrow range, and the application value of the chitosan product is improved.