The capacity fade in Lithium-ion battery (LIB) of high energy density using Si/C core-shell particle anode is one of the major barriers blocking its wide application. However, the underlying mechanism of electro-chemo-mechanical degradation remains unclear. In this study, we propose and validate a multiscale model (electrode level and particle level) with consideration of electrochemical-mechanical coupling and cohesive zone method at the particle level. The effects of charging rate, core/shell ratio, and mechanical properties of the shell on the separation and capacity fade are discussed. We discover that larger charging rate, smaller core/shell ratio, and stiffer shell can mitigate the core-shell separation gap, leading to higher capacity retention. Results shed light on the degradation mechanism of Si/C core-shell anode and provide design guidance for Si/C anode materials in minimizing the capacity fade and safe battery charging/discharging strategy.