In the optical transport network, the high-dimension equiprobable constellation is used to increase the information transmission rate and the forward error correction code is used to improve the system performance. However, adopting the equiprobable constellation will lead to some information loss according to the Shannon theory and the performance of the FEC code needs to be strengthened. Based on this, unequiprobable constellation is designed to reduce the information loss by optimizing the probability of the first bit taking the value zero or one. In order to further improve the system performance, polar code, an advanced channel code with good performance and low decoding computation complexity, is combined with the probabilistic shaping to realize the unequiprobable constellation in this paper. The simulation results show that the unequiprobable constellation can provide nearly 1dB gain compared to the equiprobable constellation.