The ultrafast photoinduced structural change dynamics of a prototypical Cu(I) complex, namely, [Cu(dmp)(2)](+) (dmp = 2,9-dimethyl-1,10-phenanthroline), is investigated based on the theoretical analysis of static and dynamical calculations at the all-atomic level. This work mainly focuses on the intriguing structural flattening features of [Cu(dmp)(2)](+) occurring in the metal-to-ligand charge transfer singlet excited state ((MLCT)-M-1) on the sub-picosecond timescale. Our estimated time constant (similar to 675 fs) of this "flattening'' motion is in good agreement with recent experimental values. The full-dimensional excited-state nonadiabatic dynamic simulation provides a direct view of the ultrafast photoinduced events of [Cu(dmp)(2)](+), especially, the structural flattening mechanism on the S-1 state. Several molecular motions (such as Cu-N stretching, the motion of the substituted groups etc.) with distinguishable time scales are involved in the flattening dynamics. The Fourier transformation of the time-dependent oscillation of the Cu-N bond and the N-Cu-N bond angle provides consistent conclusions with the experimental spectrum analysis. These dynamics details imply that various nuclear motions are strongly coupled in the high-dimensional excited-state potential energy surface responsible for the geometrical evolution of [Cu(dmp)(2)](+). This work provides us a unique fundamental understanding of the ultrafast photoinduced excited-state nonadiabatic process of Cu(I) complexes and their derivatives, which should have potential impacts on various research fields, such as photo-catalysts, dye-sensitized solar cells (DSSCs), and organic light emitting diodes (OLEDs).