The invention provides a preparation method of high-molecular-weight polyacrylamide. The method includes the steps of performing segmented polymerization to acrylamide and an initiator in a high-pressure kettle with a supercritical CO2 medium to obtain high-molecular-weight polyacrylamide; and then performing supercritical CO2 fluid extraction to extract small-molecular monomers which are incompletely reacted to obtain high-purity polyacrylamide. The product is pure and is free of follow-up treatment. The reaction medium is toxic-free and environment-friendly. The preparation method is economic and practical, and is green and environment-friendly.