A decoding method, a memory storage device and a memory control circuit unit are provided. The decoding method includes: reading a target physical unit based on a first read voltage level; performing a first decoding operation; reading an authentication physical unit based on a first candidate voltage level to obtain first assistance data and reading the authentication physical unit based on a second candidate voltage level to obtain second assistance data if the first decoding operation fails; obtaining a first estimation parameter according to the first assistance data and authentication data and obtaining a second estimation parameter according to the second assistance data and the authentication data; determining a second read voltage level according to the first estimation parameter and the second estimation parameter; and reading the target physical unit again based on the second read voltage level. Accordingly, the decoding efficiency may be improved.