Coast-down　time　measuring　system　can　measure　the　time　when　speed　values　of　Chassis　Dynamometers　for　Automobile　Emissions　Testing　change　from　initial　velocity　to　end　velocity　under　different　load　in　a　given　speed　range.　This　coast-down　time　will　represent　characters　of　Chassis　Dynamometers　for　Automobile　Emissions　Testing　such　as　Dynamometer　inertia　weight,　constant　load　coast-down　time,　variable　load　coast-down　time,　Chassis　Dynamometers　for　Automobile　Emissions　Testing　can　be　calibrated　by　comparing　these　coast-down　time　with　theoretical　values.　Working　principle　of　coast-down　time　measuring　system　is　a　photoelectric　sampler　of　coast-down　time　measuring　system　collects　speed　values　information　of　Chassis　Dynamometers　for　Automobile　Emissions　Testing.　Then,　determine　speed　values　through　calculation.　When　the　velocity　is　equal　to　initial　velocity　V-b　value,　measuring　system　starts　a　timer.　When　speed　reaches　end　velocity　V-e　value,　measuring　system　stops　the　timer.　So　time　interval　is　coast-down　time　from　V-b　to　V-e.　It　is　obvious　that　accuracy　of　coast-down　time　measurement　lies　on　whether　start　and　end　velocity　can　be　judged　accurately　and　determined　quickly,　whether　accuracy　of　timing　can　be　admitted,　and　lies　on　how　long　these　can　be　done.　Coast-down　time　measuring　system　with　photoelectric　sampler　proposed　in　this　paper　involves　all　the　above　factors,　as　well　as　improves　measurement　accuracy　of　coast-down　time.