In　order　to　study　the　effect　of　rotor　spacing　on　the　aerodynamic　performance　of　small-scale　coaxial　rotor　in　hover,　a　platform　is　designed　to　test　the　aerodynamic　characteristics　of　the　coaxial　rotor　system　with　different　rotor　spacing　ratios　h/r　of　0.32,　0.38,　0.45,　0.51,　0.58,　0.65　and　0.75.　Rotor　thrust　and　power　obtained　from　measurement　with　different　rotary　speeds　are　used　to　estimate　the　aerodynamic　characteristics　of　the　rotor　system.　In　the　meantime,　the　uncertainty　of　the　thrust　coefficient,　power　coefficient　and　power　loading　(PL),　and　the　calculated　errors　are　all　less　than　2%.　Distributions　of　streamline　and　pressure　with　different　rotor　spacing　ratios　are　obtained　from　numerical　simulation　to　analyze　the　interference　between　top　and　bottom　rotors.　Finally,　in　comparison　with　test　results,　the　aerodynamic　configuration　is　optimized　with　the　rotor　spacing　ratio　of　0.38.　The　results　prove　that　the　performance　of　coaxial　rotor　in　hover　can　be　improved　with　proper　rotor　spacing.　With　certain　rotor　spacing,　aerodynamic　performance　at　higher　rotational　speed　is　more　prominent　when　the　interference　between　the　rotors　is　beneficial　for　the　aerodynamics,　and　greater　power　loading　is　obtained　at　lower　rotational　speed　with　smaller　interference.　©　2017,　Editorial　Department　of　JOURNAL　OF　EXPERIMENTS　IN　FLUID　MECHANICS.　All　right　reserved.