How to measure the resolution of a reconstructed 3D density map is an important problem of the Single-Particle Reconstruction (SPR) of cryo-EM images. It plays a critical role for promoting methodology development of SPR and structural biology. Due to there is no benchmark map in a new structure generation, how to realize the resolution estimation of a new map is still an open question. We proposed a new self-reference-based resolution estimation protocol SRes, which only requires a single reconstructed 3D map for the purpose of resolution measurement. The core idea in SRes is performing a multi-scale spectral analysis on the map through multiple size-variable masks segmenting the map. The new SRes approach has provided a new routine for measuring the resolution from a single density map. This study is published in Yang et.al, Journal of Chemical Information and Modeling, 2018.