Various flipping operations can significantly improve the error-correction performance of polar codes based on successive cancellation (SC) and SC list (SCL) decoding algorithms. In this letter, we propose an early-stopping strategy based on a novel relative partial path metric (RPPM) for SC/SCL flip algorithms to reduce the decoding latency and complexity in the low signal-to-noise ratio (SNR) region. By introducing a dynamic threshold, the proposed strategy can delete as many redundant decoding operations as possible while keeping the performance deterioration in a tolerant region. Simulation results show that compared to the original SC/SCL flip algorithms, the proposed early-stopping strategy can achieve significant reductions in average complexity across various code lengths and rates, especially in low SNR regions.