Aiming at the rotational inertia on-line measurement of the air-floating shaft system in the sine torque standard device, a non-contact flat spiral spring is designed according to the mechanical standard .Because the number of turns of the spring is less than the recommended turns in the design standard, the finite element analysis software ANSYS is applied to analyze the characteristic curve of the spring to verify whether the spring can stably provide sufficient reaction torque to drive the air shaft twisting. Experiments show that the results have good consistency and high accuracy by using the spring to measure the rotational inertia, which can meet the measurement requirements.