Poor lateral-directional stability due to the absence of vertical stabilizer is a great risk to the aircraft with a flying wing layout. In this paper, an unmanned aerial vehicle with this kind of configuration is chosen as the research object. A three-dimensional model of the unmanned aerial vehicle is established, and then the sensitivity analysis is performed to obtain the effects of main aerodynamic shape parameters on lateral-directional flying quality. The results show that the roll mode and spiral mode of the aircraft meets the requirements of Level 1 flying quality in MIL-F-8785C. But the Dutch roll mode is generally divergent, which means that the flying quality of the aircraft is unacceptable. Thus it can be seen that the Dutch roll mode is the key to the dynamic stability of the aircraft. Further studies show that increasing the value of the wing aspect ratio or decreasing the values of the dihedral angle and torsion angle is useful for improving the Dutch roll mode. It is valuable to reveal the influence mechanism of aerodynamic shape parameters on lateral-directional flying quality for the design of flying wing aircraft.