In this work,an innovative pyrolysis reactor for waste plastics pyrolysis was developed based their phase change characteristics during pyrolysis.Thermoplastics such as polyethylene (PE) and polypropylene (PP) films recycled from municipal solid wastes (MSW) are characterized with regular phase change during their pyrolysis which include melting (from solid to liquid),volatile evaporating (from liquid to gas) and coke formation with the evaporation being the key step.