The invention provides a secure coding method based on polarization codes under a weakened wiretap channel. The method comprises the steps of 1, combining and dividing channels; 2, sequencing the channels; 3, picking an information bit channel set and a fixed bit channel set; 4, determining secure channels and unsecure channels; 5, updating a "key"; 6, obtaining a ciphertext sequence; 7, generating a sequence to be coded; 8, generating a coding matrix; 9, coding; 10, modulating; 11, passing the channels; 12, demodulating; 13, decoding; 14, finding out the "key" and ciphertext; 15, updating the "key"; 16, obtaining an estimated sequence of decrypted unsecure channel sent information; and 17, reducing an information sequence sent by a sender. The information sent by the unsecure channels is encrypted by adopting the information sequence generated by the secure channels, so that the encryption key is unlikely to decrypt, and the security of the system is improved, wherein both the secure channels and the unsecure channels are used for sending information bits, so that the channel resources are saved.