To study the influence of external store weapons on longitudinal aerodynamic characteristics of unmanned helicopter, the aerodynamic characteristics before and after installing the external store weapon are calculated by Reynolds average Navier-Stokes (RANS) method with different flight speeds, weapon installation angles and loading conditions. Then the aerodynamic characteristics of high speed flight state are compared with the wind tunnel test results that before installing the external store weapon. The results show that the influence of external store weapons with cantilever beam on the drag is relatively large, the changes of flight speed and weapon angle have little influence on aerodynamic characteristics, and the influence of loading conditions on the longitudinal aerodynamic characteristics of unmanned helicopter is small. The results can provide a reference for the choice of suitable mode of weapons external store and launch for armed unmanned helicopter.