Single-atom manipulation has emerged as an effective strategy for enhancing the photocatalytic efficiency. However, the mechanism of photogenerated carrier dynamics under single-atom modulation remains unclear. Combining first-principles calculations and non-adiabatic molecular dynamics simulations, we systematically studied carrier transfer and recombination in the oxygen reduction reaction of single-atom-doped C3N4 systems. Unlike the conventional two-step process, where single atoms trap photogenerated carriers before transferring them to reactants, our findings reveal a direct one-step electron transfer process, where single-atom-induced hybridization states facilitate the direct trapping of hot carriers by reactants from photocatalysts. Specifically, photogenerated electron transfer time through the one-step process is 237 and 325 fs for Sb and Cu single-atom-doped systems, respectively, considerably faster than the two-step process (hundreds of picoseconds). Moreover, these systems exhibit a nanosecond-level photogenerated carrier lifetime, driving a high photocatalytic efficiency. This study elucidates the carrier dynamics in single-atom photocatalysts, facilitating the screening of high-performance photocatalysts.