Theoretical and experimental studies have been made about the electrical characteristics of Hg1-xCdxTe n+-p photodiodes with x=0.5 at room temperature and at hydrostatic pressures up to 2GPa. The observed "anomalous" pressure dependence of the current-voltage characteristics of p-n junctions, which exhibits in the small bias region, has been satisfactorily explained using a theory which takes into account the pressure dependence of deep levels and its effect on deep level-assisted tunneling current. By fitting the dark current to this theory, two deep levels, D1(=Ev + 0.75Eg) and D2( = Ev+0.5Eg), as well as their respective electron and hole lifetimes, were obtained. The pressure coefficients of deep levels in undoped Hg1-x Cdx Te were also obtained.