The conformer population and the relative excitation efficiency of each conformer, assessed by theoretical and experimental examinations, enabled us to precisely control the chemo- and diastereoselectivities of the competitive photocyclization/rearrangement reaction of a chiral donor-acceptor (D/A) dyad by irradiation wavelength, solvent polarity, and reaction temperature. Manipulating the ground- and excited-state conformer equilibria is essential for critically controlling the intramolecular D/A system, in sharp contrast to the rather divergent excited species involved in relevant intermolecular systems.