The present invention relates to thermoplastic polymer foaming technology, and is especially solid CO2 supercritical foaming process. The solid CO2 supercritical foaming process includes mixing sodiumbicarbonate and citric aid powder in certain proportion into plastic grains, heating the plastic grains to smelt into liquid with sodium bicarbonate and citric aid powder, increasing the temperatureand pressure for sodium bicarbonate and citric aid to react chemically releasing CO2 in supercritical state and dissolved in liquid plastic, and lowering the temperature and pressure to foam and formthe plastic. The process is simple, and has easy control on CO2 amount, high CO2 diffusion rate and dissolving rate, high production efficiency, low cost and high safety.