Element doping plays a key role in achieving desired properties of semiconductor nanocrystals. In the energy-state landscape the doping-induced localized impurity states (LIS) can bring on significant modification of photoelectrochemical effects. It is difficult to retrieve information regarding the doping-induced LIS. Here we report on such information gleaned on a prototypical system of CdS nanocrystals slightly doped with In3+, through joint observations from photoluminescence (PL) and ultrafast transient absorption (TA) spectroscopy. The nonradiative nature of the In-doping induced LIS is revealed by PL. The TA observations, with a set of control experiments, enable us to capture a picture of the photoexcited electron dynamics and unravel the photoexcited electron reservoir (PEER) effect associated with the In-doping induced band gap LIS. This work establishes a fundamental, mechanistic understanding of the significant impact of element doping on the photoexcited electron dynamics in this model system, offering useful inputs for relevant material design and applications.