Lowering　the　amount　of　excess　air　is　believed　to　increase　the　density　of　the　air-fuel　mixture　and　help　improve　the　combustion　rate　for　compression　ignition　engines.　This　paper　proposes　an　approach　of　adding　a　throttle　body　at　the　intake　pipe　to　control　the　excess　air　ratio　with　reduction　of　air　supply　to　achieve　a　better　balance　between　the　power,　emissions　and　fuel　efficiency　at　medium　and　low　load　of　a　natural　gas　dual-fuel　diesel　engine　converted　from　a　conventional　diesel　engine.　Various　experiments　in　both　pure　diesel　and　dual-fuel　mode　under　intermediate　engine　speed　are　performed　with　the　proposed　critical　method　of　excess　air　ratio　control.　The　experimental　results　reveal　that　better　excess　air　ratio　is　very　beneficial　for　the　power　output　and　brake　specific　energy　consumption　in　dual-fuel　combustion　under　medium　and　low　load　conditions.　Moreover,　the　substitution　rate　can　reach　as　high　as　40%　under　low　load　conditions　with　throttle　control.　©　2018　by　the　authors.　Licensee　MDPI,　Basel,　Switzerland.　This　article　is　an　open　access　article　distributed　under　the　terms　and　conditions　of　the　Creative　Commons　Attribution　(CC　BY)　license　(http://creativecommons.org/licenses/by/4.0/).