p-Methoxy methylcinnamate (p-MMC) shares the same molecular skeleton with octyl methoxycinnamate sunscreen. It is recently found that adding one water to p-MMC can significantly enhance the photoprotection efficiency. However, the physical origin is elusive. Herein we have employed multireference complete active space self-consistent field (CASSCF) and multistate complete active-space second-order perturbation (MS-CASPT2) methods to scrutinize the photophysical and photochemical mechanism of p-MMC and its one-water complex p-MMC-W. Specifically, we optimize the stationary-point structures on the (1)pi pi*, (1)n pi*, and S-0 potential energy surfaces to locate the (1)pi pi*/S-0 and (1)pi pi*/(1)n pi* conical intersections and to map (1)pi pi* and (1)n pi* excited-state relaxation paths. On the basis of the results, we find that, for the trans p-MMC, the major (1)pi pi* deactivation path is decaying to the dark (1)n pi* state via the in-plane (1)pi pi*/(1)n pi* crossing point, which only need overcome a small barrier of 2.5 kcal/mol; the minor one is decaying to the S-0 state via the (1)pi pi*/S-0 conical intersection induced by out-of-plane photoisomerization. For the cis p-MMC, these two decay paths are comparable (1)pi pi* deactivation paths: one is decaying to the dark (1)n pi* state via the (1)pi pi*/(1)n pi* crossing point, and the second is decaying to the ground state via the (1)pi pi*/S-0 conical intersection. One-water hydration stabilizes the (1)pi pi* state and meanwhile destabilizes the (1)n pi* state. As a consequence, the (1)pi pi* deactivation path to the dark (1)n pi* state is heavily inhibited. The related barriers are increased to 5.8 and 3.3 kcal/mol for the trans and cis p-MMC-W, respectively. In comparison, the barriers associated with the photoisomerization-induced (1)pi pi* decay paths are reduced to 2.5 and 1.3 kcal/mol for the trans and cis p-MMC-W. Therefore, the (1)pi pi* decay paths to the S-0 state are dominant relaxation channels when adding one water molecule. Finally, the present work contributes a lot of knowledge to understanding the photoprotection mechanism of methylcinnamate derivatives.