Space charge in insulation material of high voltage direct current cable is closely linked with the ageing degree of the materials，and it is an interesting topic if one can find some characteristics to indicate the ageing of the material from the space charge measurement. In this paper，the effects of ageing on space charge characteristics of low density polyethylene (LDPE) are investigated by the pulsed electro-acoustic (PEA) method. A space charge trapping model is proposed base on distribution of discrete traps，and the characteristic parameters in the model including trap depth and trap density are calculated. The results show that the relatively shallow traps in the samples transform to the relatively deep traps as the degree of electrical ageing increases. Furthermore，the relationships between ageing degrees and trap depth and trap density are obtained. The information of the trapping parameters in our model provides a good reference for diagnosing the ageing degree of polymeric materials.