In　China,　where　in-use　heavy-duty　diesel　vehicles　producing　NOx　and　particulate　emissions　for　air　pollution　are　required　to　undergo　emission　spot　inspection　at　check　stations,　it　is　significant　to　adopt　a　simple　method　to　evaluate　emissions　due　to　traffic　jams,　especially　in　big　cities.　To　realize　convenient　vehicle　emission　spot　inspection,　this　paper　investigates　the　effects　of　exhaust　temperature　on　the　Selective　Catalytic　Reduction　(SCR)　de-NOx　conversion　efficiency　and　presents　a　method　for　evaluating　the　SCR　operation　state　based　on　ammonia　storage　characteristics.　The　paper　proposes　a　real-road　driving　test　procedure　and　verifies　it　by　measuring　NOx　from　a　heavy-duty　diesel　vehicle　in　an　on-road　driving　test.　The　results　show　that　the　SCR　de-NOx　efficiency　experiences　three　operation　states.　In　state　I,　SCR　works　and　injects　urea,　resulting　in　high　de-NOx　efficiency　(＞90%);　state　III　occurs　during　the　cold,　starting　with　the　lowest　de-NOx　efficiency　(＜50%)　due　to　a　lack　of　NH3;　and　state　II　is　a　transition　stage　caused　by　the　ammonia　storage,　with　a　certain　conversion　efficiency　(50-90%)　when　SCR　de-NOx　efficiency　linearly　relates　to　exhaust　temperature.　Whether　the　SCR　works　normally　can　be　judged　based　on　the　SCRs　operation　states.　This　method　is　simple　and　easy　to　implement　in　different　SCR　operation　states,　and　it　is　effective　and　repeatable.