Abstract: It is difficult to detect the roundness error of large rotary bodies. In this paper, the non-contact soft-measurement technology is used to measure the measured signal of large-scale rotary body. Based on the wavelet transform denoising filter method, the roundness error signal in the measured signal is quickly decomposed and extracted, and the roundness error of the large-scale rotary body is calculated by the roundness error evaluation. The results show that the method can effectively extract the elementary error signal in the measured signal of large-scale rotary body. The roundness error is 0.32 mm by calculation. The method provides a certain theoretical basis for the calculation of roundness error of large-scale rotary body, has a certain practical application.