In order to investigate the validity of the scale-down experiment on gas discharge, the discharge in argon at low pressure is numerically simulated with scale-down discharge gap based on the conjecture of discharge similarity that if the product of gas pressure p and gap length d is kept constant, p1d1=p2d2, and the spatial distributions of the reduced field E/p along these two gaps are the same, the gas discharges in these two discharge gaps would be similar. In the simulation, three scale-down discharge gaps are used. Gap A is 30 mm long and works at a pressure of 1 Torr (1 Torr=133.322 Pa). Gap B is 15 mm long at 2 Torr and gap C 10 mm long at 3 Torr. The results show that the discharges in these three gaps are glow discharges with a cathode fall layer. The values of thickness of the cathode fall layer, dC, for gaps A, B and C are 2.71 mm, 1.35 mm and 0.87 mm, respectively, which corresponds to more or less the same value of pdC≈2.70 Torr·mm that is close to the lowest point of Paschen curve of argon where pd≈2.86 Torr·mm. The proportionalities of the parameters (working voltage, electric field, current density, electron density and ion density) between the discharges in the scale-down gaps are found to be in good agreement with those determined by the discharge similarity. It is concluded that the conjecture of discharge similarity is correct for the glow discharge in argon in the scale-down gap.