DIW　(Dynamometer　Inertia　Weight)　of　＇chassis　dynamometers　for　automobile　emissions　testing＇　is　a　total　inertia　weight　of　all　rotating　components　in　the　chassis　dynamometer.　Total　Inertia　weight　is　a　inertia　device　simulates　translational　and　rotational　kinetic　energy　of　a　traveling　vehicle　which　is　equivalent　to　the　mass　of　the　car.　DIW　is　an　important　technical　indicator,　whether　it　is　accurate　or　not　will　affect　the　calibration　of　all　technical　characteristics　of　＇chassis　dynamometers　for　automobile　emissions　testing＇.　In　this　paper,　first　of　all,　we　introduce　a　new　method　to　measure　the　Dynamometer　Inertia　Weight,　that　is,　coast-down　testing　with　twice　constant-force　loading　method,　and　we　derivate　a　formula　to　calculate　the　Dynamometer　Inertia　Weight　from　kinetic　energy　conservation　law.　Secondly,　we　have　done　a　lot　of　coast-down　testings　with　twice　constant-force　load　method.　After　analyzing　the　data,　we　found　out　the　factors　that　affect　the　testing　processes　and　the　accuracy　of　testing　results.　Finally,　after　comparing　twice　constant-force　loading　method　with　the　take-down　flywheel　method　and　twice　constant-power　loading　method,　we　know　that　this　coast-down　testing　with　twice　constant-force　loading　method　is　better　than　other　methods　in　stability　and　repeatability　and　testing　data　will　be　more　accurate.　It＇s　a　accurate　and　convenient　way　to　measure　the　DIW　of　＇chassis　dynamometers　for　automobile　emissions　testing＇.