Quantum key distribution guarantees the unconditional security of communication by combining with the one-time pad encryption scheme. However, the error correction and privacy amplification of quantum key distribution post-processing result in high processing delay, influencing the final secret key generation rate and the practicality of quantum key distribution system. Therefore, this paper proposes a one-step post-processing algorithm based on polar codes for quantum key distribution. By analyzing the channel capacity of the two legal communicators and the eavesdropper respectively under wyners wiretap channel model, this paper designs a codeword structure of polar codes which could satisfy the reliability and security for quantum key distribution post-processing, so that the error correction and privacy amplification could be completed synchronously in every encoding and decoding. Combining the two processing steps into one, it reduces the system complexity and the processing delay. Experimental results show the proposed algorithm could satisfy the reliability condition that bit error rate less than 10-7 after error correction and the security condition that the information eavesdropping less than 10-14 in the quantum bit error rate [0,0.08], and the decoding throughput could achieve 3Mbps and 86Mbps with parallel decoding under the code length 220.