The RCS (Rankine cycle system) used to recover the exhaust gas energy from internal combustion engines has been regarded as one of the most promising ways to achieve higher efficiency. However, it is a big challenge to keep the RCS still in good performance under variable driving cycle. This paper aims at revealing the reasons resulting in the low efficiency under driving cycle, comparing to that under steady-state condition.The dynamic operating process of the RCS under driving cycle is analyzed, and then the RCS applied on an 11.6L heavy duty diesel engine is modeled. Based on that, the dynamic performance of the RCS under an actual driving cycle is discussed. The results indicate that the average efficiency under a piece of Tianjin bus driving cycle is as low as 3.63%, which is less than half of that (7.77%) under the rated point (1300rpm and 50%load). The reasons leading to the low efficiency under driving cycle is interpreted from three aspects. Firstly, effects of the optimizing criterions are studied. It is found that the optimal on-road efficiency does not appear at the point where the highest rated efficiency is achieved. Secondly, the effect of the rated operating point selection is taken into account. The results show that the rated efficiency favors abundant exhaust energy, but the effective operating range, on the contrary, may shrink. Finally, the frequency characteristic of the RCS is analyzed. It is difficult to control the slowly responded RCS in step with the highly-frequency changed engine operating point under driving cycle. Control objective programming and dynamic process control are two coupling and trade-off factors, which significantly affect the on-road efficiency. Two key parameters are defined to evaluate the effects respectively. In conclusion, in order to obtain high on-road efficiency, it would be better to take the rated efficiency, effective operating range and dynamic control into account synchronously in RCS design and optimization.