As　early-stage　design　decisions　are　critical　to　building　energy　efficiency,　performance-oriented　design　supporting　methods　have　been　intensively　investigated　using　various　cutting-edge　technologies.　However,　applying　such　methods　in　real　design　scenarios　is　rather　limited,　causing　their　actual　effectiveness　remains　questionable.　To　address　this　problem,　a　design　experiment　is　reported　in　this　paper,　where　41　designers　conducted　the　early-stage　design　of　an　office　building,　first　without,　and　then　with　the　help　of　MOOSAS,　a　user-friendly　tool　providing　rapid　predictions　of　building　energy　efficiency.　The　results　showed　that　after　the　use　of　MOOSAS,　the　energy　efficiency　of　the　design　outcomes　improved　significantly,　with　the　mean　energy　use　intensity　decreasing　by　10%.　Moreover,　participants　reached　new　design　possibilities　with　higher　energy　efficiency　during　their　design　explorations,　and　had　a　better　chance　to　choose　the　＂optimal＂　or　＂near-optimal＂　design　from　their　intermediate　options.　Other　positive　changes　in　the　design　process　included　a　greater　variety　of　design　explorations,　the　achievement　of　an　upward　trend　in　energy　efficiency,　and　more　comprehensive　coverage　of　the　early-stage　design　subthemes.　The　questionnaire　following　the　experiment　showed　that　76%　of　the　participants　regarded　MOOSAS　as　effective　or　even　very　effective,　whereas　56%　claimed　to　use　MOOSAS　voluntarily　in　the　future.　It　is　further　implied　that　a　more　hands-on　educational　approach　in　building　science　courses　could　be　advantageous,　while　forming　a　culture　of　regarding　building　performance　as　architects?　responsibility　and　using　the　supporting　tools　in　the　pre-design　stage　may　benefit　the　design　practice.