In this paper, the band profiles of HgCdTe (MCT) heterojunctions with the configuration of wide/narrow gap-p on narrow/wide gap-n are calculated, respectively. In the theoretical model employing a valence band offset, a new simple carrier density approximation is proposed to include the carrier degeneracy and conduction band non-parabolicity in the calculations. The effects of the location of p-n junction (zB) and the interface charge density (Q) on the band profiles of MCT heterojunctions are studied systemically, which is found to be significant and can't be neglected. With the help of the results of above research, the optimal design related to zB and Q is obtained.