This paper contributes to giving physical insights into stability of DC-link voltage control in multi-wind turbines integrated to weak grid. Wind farm stability operation will suffer from integrating to weak AC grid, due to strong coupling between the wind turbines control and grid dynamics. Multi-VSCs model is presented for DC-link voltage control stability analysis with effect of coupling among VSCs and grid strength. Based on the model, self-impact components and interaction-impact components are presented to study interactions among VSCs. Impacts of VSCs interactions on stability of DC-link voltage control are studied by investigating self-impact and interaction-impact components variations with considering grid strength, operating point and control loop bandwidth. It is found that interactions between VSC1 and VSC2 become more serious with decline of grid strength, decaying stability of VSCs. Theoretical analysis is verified with simulation of two parallel wind turbines connected to weak grid.