Lithium (Li) ion batteries are in high demand by most of portable electronic devices. Silicon (Si), owing to its large theoretical energy density (4200mAhg−1), is a promising candidate for anode material. However, loss of electrical contact caused by the pulverization that comes from the enormous volume changes during charging and discharging process hinders its application. The aim of the current work is to elucidate the possibility of using carbon nanotubes (CNTs) for the Si reinforcement to minimize the volume expansions. Firstly, ab initio simulations demonstrate that the Li ions can permeate CNT through the tube end freely under the normal charge voltage (∼3.6V). Then, molecular dynamics simulations are adopted to analyze the strength improvement of silicon nanowires (SiNWs) encapsulated by CNT. The compression simulations of SiNW were performed to reveal the expansion processes during Li ion insertion into the electrode. The results show that the smaller the SiNW is, the better the reinforcing effect is. A phase transition appears in the SiNW encapsulated by CNT during compression, which makes SiNW more ductile. The simulation results provide an effective way for the SiNW reinforcement in the Li ion battery.