The invention provides a codeword constructing method for a variable length Polar code, which belongs to the field of communication. According to the invention, a long code generation matrix is constructed by extending the generation matrix of a short code. The method comprises the steps that based on integer binary representation, the code length is expressed as the sum of the powers of two; the value of each term constituting the sum form is used as the size of the sub-matrix which makes up a new generation matrix; and finally, the new generation matrix is constructed through combo-sum combination operation. Compared with a traditional variable length Polar code construction method, the method provided by the invention has the advantages that the puncturing delete operation is not needed; the coding complexity is reduced; delay is reduced; on the decoding end, the likelihood information for the unknown bit is known, which avoids the problems of a traditional method which uses prior knowledge to estimate the unknown bit likelihood information; the error bit rate of decoding is reduced; and the system performance is improved.