Polar codes have been proved to achieve the capacity in the binary–input discrete memoryless channels(B-DMCs)with low encoding and decoding complexities.In the communication system,high-order modulation is always adopted for high spectrum efficiency.The multilevel coding(MLC)approach has been proposed that allows for a joint description of binary polar codes and 2m ?ary modulation schemes.In this paper,a study is conducted to achieve a better system performance through optimizing the constellation and the mapping based on the above MLC approach.Set partition labeling is the best way of mapping for the MLC approach and a better system performance can be achieved by optimizing the signal constellations when 8-PSK is adopted in AWGN channel.