The range-extended electric transit bus (REEbus) equipped with the auxiliary power unit (APU) using high efficient diesel engine as power source can reduce the cost of power battery and is an ideal transitional powertrain architecture to the pure electric drive. Based on chassis tests of a 12m long REEbus, fuel consumption and emission characteristics during Charge-Sustaining (CS) stage effected by temperature of the REEbus are researched. The APU of REEbus starts to work around just one point with best efficiency and lower emission when the state of charge (SOC) is too low and stop when the SOC is high, which aims to lower fuel consumption. As a result, even during CS stage, the fuel consumption of REEbus is only 22.84 L/100km. Also almost all emissions decrease dramatically and the NOx emission is only 0.68g/km, but the ultrafine-particle number increases owing to better combustion. For the engine of REEbus there are colder engine start, severer acceleration and deceleration compared to of the traditional powered buses, so during start for too much fuel injection and acceleration for lacking in inlet fresh air as well as shutdown processes of APU, there are obvious overshoots of the APU speed accompanied with sharp increases of CO, THC emissions, which is more obvious during engine start on colder condition. What’s more, on warm condition the catalyst of SCR lights off much earlier than on cold condition, contributing to significant decrease of NOx emission. So it turns that on warm condition, CO, THC, NOx emissions and the ultrafine-particle mass reduce respectively by 62.6%, 25.3%, 68.1% and 41.0% compared to on cold condition, but the ultrafine-particle number increases by 58.7%.