Constructing piezoelectret based on foamed plastic garbage is an advisable strategy for obtaining self-powered flexible electromechanical sensors with good performances. Herein, a self-powered piezoelectret sensor with basic material of low density polyethylene (LDPE) foamed plastic garbage is proposed, with characteristics of easy fabrication, excellent flexibility, and high equivalent piezoelectric coefficient d₃₃ value up to ~ 1,100 pC/N. The output stability is verified by continuously stimulating a sensor for ~ 180,000 cycles under low and high applied pressure, and the variations of peak outputs are less than 5.5%. Applications for measuring low- and high-pressure signals from human body are achieved. Assembled with a wristband, a sensor is demonstrated for detecting the human pulse waves. Moreover, real time human sitting information is wirelessly monitored with a smart chair based on 4 pixels sensors array.