The aim of this study is to investigate the kinetic characteristics of the high-density polyethylene (HDPE) waste pyrolysis process based on thermogravimetric analysis (TGA) and using a fixed bed pyrolytic reactor (FBPR) at different temperatures. In addition, the influence of material bed thickness on the yield distribution and the composition of products was examined over a temperature range of 425–550°C. A higher wax fraction was obtained in the thin bed of the FBPR bed at 425°C. With temperature above 500°C, more oil and wax products were generated in the thick bed of the FBPR. Based on the experimental study, a discrete first-order kinetic lumping model, comprising three independent parallel reactions (lumps), was developed to describe the yield distribution of gases, oil fractions, wax fractions, and solid residue, coupling with secondary cracking reactions of the wax fractions into lighter fractions (i.e., oil and gas). The overall apparent activation energy (Ea) and pre-exponential factor (A0) of HDPE pyrolysis were estimated in the FBPR. The results showed that the thickness of the bed of plastics has a pronounced influence on the pyrolysis kinetics of HDPE.