Pyrolysis of waste plastic (WP) is a promising method to solve the plastic pollution issue. WP is mainly composed of polyethylene (PE). Moreover, the products of waste polyethylene (WPE) pyrolysis could serve as high quality fuels and the feedstocks of petrochemicals. Therefore, it is essential to investigate the WPE and WP pyrolysis process. This study evaluates pure PE, WPE and WP pyrolysis kinetic parameters by the use of genetic algorithm (GA) and isoconversional methods coupled with thermogravimetric analysis (TGA), respectively. Additionally, three representative reaction models, i.e. reaction-order, extended Prout–Tompkins and Sestak–Berggren models, are investigated for obtaining the most suitable model, which could describe the PE, WPE and WP pyrolysis process more accurately. Consequently, the reaction-order model turns out to be the optimal method for appropriately describing PE, WPE and WP pyrolysis processes. Hence, the pyrolysis parameters optimized by GA are proven to be accurate and reliable, in comparison of calculated values of activation energy by isoconversional methods and experimental data. Moreover, it might be applicable of GA coupled with TGA with reaction-order model to the future industrial WPE and WP pyrolysis circumstances that have variable heating rates