To address the issue that it is difficult to detect the internal cavity of the small-size ballscrew nut and evaluate its machined surface quality due to its complex internal geometrical configuration and small aperture, a multi-scale measurement method of ballscrew nut based on the spectral confocal principle is proposed. A non-contact optical displacement sensor is used to perform indexed sectional scanning axially for the internal cavity, and three-dimensional point cloud data of the cavity surface expressed in the cylindrical coordinate system are obtained, in which the abnormal points at the interface between the raceway and inner wall are removed by using uniform interpolation filtering. Then, the denoise process is implemented by the wavelet threshold method. Based on the multi-resolution analysis characteristics of the two-dimensional wavelet decomposition, its number of decomposition layers is adaptively determined by means of the energy conservation property of the wavelet decomposition. The inner surface of the screw nut transformed into the Cartesian coordinate system unwrapped along the generatrix is dissociated at different scales, in which the low-frequency components are employed to calculate the profile structure parameters and the high-frequency components for assessing the surface roughness. The inspection results show that the proposed measurement method can evaluate not only the dimensional accuracy of the inner cavity structure but pitch error and lead angle error of the surface quality are 2 μm and 0. 016°, respectively.