Calibration of solid state nuclear track detector CR-39 was carried out with monoenergetic protons from 2×1.7 MV Tandem accelerator and 400 kV Cockcroft Walton accelerator in Beijing Normal University. To ensure the protons to have monoenergetic energy and reduce the track density to 106 cm-2, two different methods were adopted. On the tandem accelerator a slit plus rotating target plate were used to achieve the required low density irradiation while a high voltage pulse generator with pulse duration of 100 ns was used on the Cockcroft Walton accelerator. The calibration shows that the registration threshold energy of protons in CR-39 is about or a little lower than 20 keV. The response curve of track diameter vs. etching time and proton energy for protons with energy from 20 keV to 1020 keV can be used to determine the number , energy and angular distribution of the protons produced by nuclear reaction and laser plasma acceleration. Etching dynamics of high energy protons (320—1020 keV) was studied. The theoretical curve is consistent with experimental data. This method of controling the proton intensity employed in this paper could also be used in similar accelerator calibration.