In this paper, characteristics of charge trapping and detrapping in low density polyethylene under dc electric field are investigated using the pulsed electroacoustic technique. It is found that the charge decay shows very different features for the samples with different periods of applied electric field. A simple trapping and detrapping model based on two trapping levels is proposed to qualitatively explain the observation. At the same time, numerical simulation based on the above model is carried out to extract parameters (trap depths and concentration) related to the material. It is found that the space charge decaying in the first few hundred seconds, corresponding to the fast changing part of the slope, is trapped in a shallow trap with a depth in a range between 0.77 and 0.81 eV, and the trapped charge density reaches (1.1681.553) 1019 m-3 in the sample volume measured. At the same time, the space charge that decays at longer time, corresponding to the slower part of the slope, is trapped in a deep trap with a depth in a range of 0.96 and 1.01 eV, and the trapped charge density is (1.1944.615) 1018 m-3. The trap depths and charge densities of both shallow and deep traps may increase with ageing, and the parameters of two energy wells can be used as an indication of the material aging.