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