The radiative or non-radiative relaxation pathway of excited-state energy plays a crucial role in determining the feasibility and efficiency of photochemical reactions. As a key intermediate in pharmaceutical synthesis, the photophysical dynamics of naphthalene-2,6-dicarboxylic acid (2,6-NDA) have not been systematically elucidated. In this study, femtosecond transient absorption spectroscopy combined with quantum chemical calculations is employed to investigate the ultrafast dynamics of different electronic excited states of 2,6-NDA in dimethyl sulfoxide (DMSO) solution. Structural analysis reveals that electronic excitation of 2,6-NDA predominantly induces bond angle relaxation within the naphthalene core, while the geometric configuration of the carboxyl groups and the overall molecular planarity are largely preserved. Upon excitation at 328 nm and 266 nm, molecules populate different electronic states. Following excitation to high vibrational levels of the S₁ state, the molecule undergoes rapid vibrational cooling to lower vibrational levels, followed by efficient internal conversion back to the ground state. This relaxation pathway is facilitated by the fact that the initial excitation energy exceeds the energy barrier at the S₁/S₀ conical intersection. For molecules excited to highly vibrationally excited levels of the S₂ state, vibrational relaxation precedes S₂→S₁ internal conversion, after which the system ultimately returns to the ground state. However, due to substantial vibrational energy dissipation during the S₂→S₁ internal conversion process, the residual energy in the S₁ state falls below that of the S₁/S₀ conical intersection. Consequently, the subsequent relaxation rate to the ground state is slower than that observed for the pathway initiated by direct excitation to high vibrational levels of the S₁ state. These findings demonstrate that the internal conversion rate between excited states can be modulated by the initial photoexcitation energy, offering a potential strategy for designing fluorescent materials with excitation wavelength-dependent properties.