A method and an apparatus for encoding and for decoding a polar code to reduce complexity and improve speed. For encoding, information bits are obtained, an original kernel matrix is adjusted to construct one or more kernel matrices, an appropriate target kernel matrix is selected from the one or more kernel matrices, and polar encoding is performed on the information bits based on the target kernel matrix. For decoding, a to-be-decoded sequence is obtained, and the to-be-decoded sequence is decoded based on a plurality of trellises, where intermediate results obtained in different decoding stages may be reused. For example, in a (t+i)th stage of decoding, an intermediate result obtained in a tth stage of decoding is reused.