The CFD method with moving overset grid scheme is used to simulate the flow-field of a tilt-rotor aircraft during helicopter mode and conversion mode in small forward velocity flight. The wakes from different flight conditions are investigated and their effects on the horizontal tail are explored. In helicopter mode， when the aircraft is in hover or small forward velocity flight（≤4 m/s）， the rotor wakes， which form fountain flow effects due to the presence of wing， focus in the vicinity of wing and have few effects on the horizontal tail. With the increase of forward velocity， a pair of stream-wise vortexes called fountain vortexes， which dominates the flow around the horizontal tail， is formed by the interaction between the fountain flow and the freestream. When the forward velocity is further enlarged（≥16 m/s）， the edge vortexes of rotor wakes move downstream and have significant effects on the horizontal tail. The fountain vortexes are generated from the upper surface of wing； the edge vortexes are from the edge of rotor disc. Although their production mechanisms are different， both the fountain vortexes and edge vortexes cause an upwash near the horizontal tail， and therefore a pitch down moment for the aircraft. In addition， the edge vortexes of rotor wakes are weakened and the pitch down moment is reduced， with the flight condition changing from helicopter mode to fixed-wing mode.