The retarder is an important auxiliary braking device of heavy vehicles. However, the stirring air in the working wheels of the idle retarder would cause the transmission loss when the vehicle is traveling in non-braking state .For certain driving conditions, the air-friction characteristics in the working wheels of the idle retarder are analyzed first. Then the relationship between the air density and the torque produced by stirring air is studied. The thermal characteristics of the retarder in the idle condition are also concerned according to the energy flow and heat transfer. Meanwhile, the increased transmission loss caused by the rising temperature of the stirring air and its inference on the transmission stability are also studied. Finally, the optimal range of air vacuum degrees in the working wheel of the idle retarder is determined and the evaluations for the air-friction and the heat transfer characteristics are given for the vacuum degrees. The result shows that, the transmission loss could be reduced by 90% and the transmission stability is verified by kinds of driving conditions, while idle retarder could be operated in a good thermal condition.