To study the aerodynamic performance of hovering octorotor small unmanned aerial vehicles (SUAV) with different rotor spacing, the computational fluid dynamics (CFD) method is applied to analyze the flow field of an octorotor SUAV in detail. In addition, an experimental platform is built to measure the thrust and power of the rotors with rotor spacing ratios L/D of 1.0, 1.2, 1.4, 1.6, and 1.8, sequentially. According to the theory of momentum, rotor aerodynamic performance is obtained with qualitative analysis. Further analysis with numerical simulation is presented with the flow field of the octorotor SUAV, the vorticity distribution, velocity distribution, pressure distribution, and streamline. The results show that the aerodynamic performance varies with the rotor spacing. Specifically, the aerodynamic performance is poor at L/D = 1.0, which is accompanied with strong interaction of wake and tip vortexes and interaction with each other. However, the aerodynamic efficiency is much improved with a larger rotor spacing, especially achieving the highest at L/D = 1.8, which is considered to be the best rotor spacing ratio for this kind of octorotor SUAV.