Herein, hierarchical porous carbon derived from polyethylene waste (plastic bags etc.) has been synthesized by ball-milling and carbonization with a flame-retardant agent, basic magnesium carbonate pentahydrate (MCHP: 4MgCO(3) center dot Mg(OH)(2) center dot 5H(2)O). The presence of MCHP not only provides in-situ MgO template during pyrolysis, but also greatly enhances the thermal stability of polyethylene for carbonization. After subsequent NH3 activation, the polyethylene waste derived carbon (PE-HPC-900NH(3)) shows a high surface area and a unique property of meso-porosity, which contribute to the excellent capacitive performance. Remarkably, the obtained PE-HPC-900NH(3) electrode displays a relatively high specific capacitance with excellent cycling stability (about 97.1% of capacitance retention after 10,000 cycles at 2 A g(-1)). A high energy density of 43 Wh kg(-1) can be achieved for the PE-HPC-900NH(3) symmetrical supercapacitor at an extremely wide voltage of 4 V in EMIMBF4 due to the high purity and low ratio of O/N. This work provides a promising approach to disposal of waste plastics and opens new applications in various energy fields.