In indirect-drive inertial confinement fusion experiments with vacuum or low-gas-fill hohlraums, collisionless electrostatic shocks can be launched in the hohlraum wall/alblator (or the low-density fill-gas) interpenetration region, which reflect ions at twice the shock velocity. A low-energy Thomson ion spectrometer was designed to measure the energy spectra of the reflected ions on the order of 10 keV generated by nanosecond-laser-driven non-relativistic collisionless electrostatic shocks. Monte Carlo simulations of ion measurement were carried out with Geant4 modeling to evaluate the influence of residual gas in the vacuum chamber and gas jet on the measurement of the low-energy ions. Simulation results show that the residual gas in the vacuum chamber causes the signal of D ions on the order of 10 keV to broaden in both the electric and magnetic deflection of the spectrometer. The broadening of the electric deflection will increase the risk of overlapping of ion spectral lines of different charge-to-mass ratios, while the broadening of the magnetic deflection will lead to the broadening of the energy spectra of the ions. The gas jet causes the ion signal to move and tail into the lower energy region, causing the measured ion spectra to deviate from the actual energy spectra of the reflected ions.