An rotational inertia adjusting device for the experimental platform of an elevator system comprises a rotary disc with adjusting sheets. Two ends of the main shaft of the rotary disc are installed on antifriction bearings respectively, wherein one end is connected with a transmission system of the experimental platform of the elevator system through a divided shaft coupling. The transmission system drives the rotary disc to rotate, the adjusting sheets are installed on the two side surfaces of the rotary disc through bolts, and the rotational inertia of the experimental platform of the elevator system can be adjusted by adding or reducing the numbers of adjusting sheets. The apparatus has the advantages of simple structure and operation, high safety and reliability and convenient installation and dismounting. The rotational inertia of a system can be rapidly adjusted according to the experimental parameters when safety control experiment of an elevator system is conducted, so that the elevator system experiment is more similar to on-site use conditions. The obtained experimental data is more accurate and reliable.