A long-term variation of the inner zone high-energy proton environment at low orbits was investigated by DSTM using the adiabatic approximation of charge particle motion with NASA standard radiation models as reference states. The DST results show that over the past three decades the fluxes of high-energy protons at (1000 km in the South Atlantic Anomaly (SAA) noticeably increased, the center region of proton SAA apparently moved westward and expanded. Calculations of the L-shell averaged lifetime of high-energy protons indicate that the DST provides a reasonable means for estimation of the secular variation of inner zone proton environment.