The invention provides a source coding method based on polar codes. The method includes the steps that firstly, a symbol value set of source restoring signals is determined, through an optimal symbol-to-bit mapping rule, a source symbol sequence is mapped into a bit soft information sequence in a serial mode, polar code serial offset coding is conducted on the bit soft information sequence, a coding path with the maximal reliability value is found, a bit sequence corresponding to the coding path is output, and finally the most important part of bits are selected from the bit sequence through a reliable bit set to serve as a coding result. The search width of the coding path is increased through a serial offset coding method, and the selection probability of the correct coding path is increased. The reliable bit set can assist a coder in distinguishing the most important bits in the coding process, the most important bits are output, and then the coding length can be compressed. In addition, the method can flexibly configure coding parameters and the symbol-to-bit mapping rule, the calculation complexity is low, and the coding performance is excellent.