In this paper, the instability mechanism of a grid-connected dc microgrid is revealed considering the impact of voltage positive feedback of selected frequency(VPFOSF) islanding detection scheme by developing the output impedance model of voltage source converter(VSC) and distributed generators(DGs). Then, the stability region is derived to guide the design method of a damping compensator. On the basis of this, an active-damping stabilization method is further proposed to mitigate instability caused by the VPFOSF loop. Finally, the effectiveness of the proposed control method is verified by detailed time-domain simulation results.