The UAV features negative stagger and wing tip gap of 5% wing span, and strong interference exist between components of the wing systems. Reynolds Averaged Navior-Stokes scheme is adopted to analyze its drag polar, stall and pitching moment characteristics. The research revealed that the induced drag is reduced by 9% compared with equal mono-wing, which is close to the theoretical result; The trimmed maximum lift coefficient drops significantly due to the requirements of static stability and trim, which are realized by -4 degree negative incidence angle of aft wing; Nose up pitching moment is observed at large angle of attack after stall, which attributes to the forward wing tip stall due to its sweptback, as well as the decreased efficiency of the aft wing in the wake of the forward wing.