A bit tagging method, a memory control circuit unit and a memory storage device are provided. The method includes: reading first memory cells according to a first reading voltage to generate a first codeword and determining whether the first codeword is a valid codeword, and the first codeword includes X bits; if not, reading the first memory cells according to a second reading voltage to generate a second codeword and determining whether the second codeword is the valid codeword, and the second codeword includes X bits; and if the second codeword is not the valid codeword and a Yth bit in the X bits of the first codeword is different from a Yth bit in the X bits of the second codeword, recording the Yth bit in the X bits as an unreliable bit, and Y is a positive integer less than or equal to X.