To address the issue where the traditional critical set (CS) of polar codes includes bit positions that may result in the elimination of the correct path, we proposed a successive cancellation list (SCL)-Flip (SCLF) decoding algorithm with an optimized critical set (OCS-SCLF). In the proposed algorithm, the initial critical set was taken as the starting point, and the Gaussian approximation principle was applied to estimate the reliability of the polarized subchannels. The selection rules of the initial critical set were then modified to address the incompleteness of the CS in SCL decoding. The OCS was constructed and arranged in ascending order of channel reliability, effectively improving the flipping accuracy and reducing the number of re-decoding attempts. The simulation results showed that the proposed algorithm achieved better performance gains while significantly reducing the number of flips.