Abstract: The driving technology of a high-enthalpy shock tunnel determines the wind tunnel's total enthalpy and total pressure. The heavy piston compression heating technology is the key driving technology of high-enthalpy shock tunnels, which shows strong driving performance and high operational flexibility. Given the difficulty in the soft landing of a large-size heavy piston caused by the launch efficiency of the heavy piston, the friction between the heavy piston and the wall, the diaphragm breaking, etc., the influencing factors of the actual moving process of the heavy piston are analyzed by combining theoretical analysis, dynamic grid numerical simulation and experimental verification, and the tuned operation method of the heavy piston is established. The FD-21 high enthalpy shock tunnel with the world's largest free piston drive ensures the safe operation of the piston with a mass of hundreds of kilograms in the compression tube with a length of 75 m. It obtains a stable driving pressure, providing corresponding test states for different simulation requirements. This paper studies the soft landing process of heavy pistons with a 205 kg and 275 kg mass in compression tubes when the maximum speed exceeds 350 m/s and 450 m/s, respectively. The soft landing of heavy pistons in large free pistons shock wind tunnels is solved. The constant pressure test conditions with a total pressure of 15 MPa and a total temperature of 3450 K, as well as a total pressure of 45 MPa and a total temperature of 4845 K at the end of the compression tube, are obtained.