The distribution of trap levels in the low density polyethylene (LDPE) was studied by means of photo-stimulated discharge (PSD). And two scanning modes, continuous scanning and step scanning, were discussed. It was pointed out that there may be some problems because of the incomplete detrapping of the trapping charges for the continuous scanning. The principle and the experimental process for the step scanning were presented. It is found that there is a linear relationship between the logarithm of PSD current produced under the irradiation of the monochromatic light and the time, which is in accordance with the theoretical analysis and indicates that the retrapping effect of the carriers can be neglected. The charges are trapped in the levels of 4.36—6.22 eV in LDPE, the main part of which were trapped in the energy levels of 4.78—5.18 eV.