In this paper, 2-D numerical simulation was performed on HgCdTe nBn detectors to study the photoelectric characteristics. We changed the parameters of each layer of HgCdTe nBn structure, including thickness, doping concentration and Cd composition, to study the variation of performance of nBn devices. We also proposed an optimal nBn structure for achieving high performance of HgCdTe nBn Detectors.