Objective Falling is a great threat to the physical and mental health of the elderly. In view of the problems about the complicated structure of the existing fall protection device and the unsafe way to inflate the gas, this paper designs a mechanical device based on detecting and tripping inflate automatically, which can help the elderly avoid injuring by the air bag inflatable protection when they accidentally fall.Methods The protective device was composed of a compressed carbon dioxide gas cylinder, an inflatable mechanism and a protective airbag. Firstly, the protective module used CO2 as aeration source, and the transmission mechanism used a high-power steering gear to drive the rotation of the coaxial cam. With the cam rotating, the puncture needle with linear motion punctured the gas cylinder. Secondly, The tensile machine was used to test the puncture needle 's strength of piercing cylinder, and then selected the appropriate torque servoaccording to the maximum piercing force and the cam drive. Finally, the gas-kinetic isentropic flow formula was used to predict the airbag inflation time. Results The students whose height was between 163 cm and 174 cm were tested to imitate the gait of the elderly about performing forward, backward and side fall tests respectively, and measured these three ways to fall in the time of (865±41. 49) ms, (788±53. 56) ms, and (852 ± 65. 37) ms respectively; simultaneously, the air balloon inflation was (441± 29. 6) ms by simulating the airbag inflation process. Comparing the fall time obtained by the experiment with the inflating time of the airbag, we know that there was ample time for the airbag to inflate during the fall of the elderly, and verified the feasibility of using compressed carbon dioxide to realize the fall protection with the airbag inflation. Conclusions The fall protection device based on the fall detection can achieve the fall protection and reduce the damage caused by the fall. The device has the advantages of simple structure, high reliability and low price, and is suitable for popularization and application.