Developing high-efficiency and low-cost multifunctional electrocatalysts is the core of unitized regenerative fuel cells (URFC), yet it remains a great challenge. Here, by performing first-principles calculations, we report the atomic-level electrocatalytic activity mechanism of 3d, 4d, and 5d monoatomic transition metals (TM) bound to the 1T-MoS2 monolayer for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Their structural stabilities are evaluated via the formation energy, elastic constant, and molecular dynamics simulations. Compared with the Co-N4-C single atom catalyst (SAC), the resulting Pd@1T-MoS2 SAC exhibits better bifunctional catalytic activity, with OER overpotential as low as 0.43 V and an ORR overpotential of 0.40 V. The dual volcano plot demonstrates that the bifunctional OER and ORR activities of Pd@1T-MoS2 originate from the neither strong nor weak OH* adsorption and the suitable d-band center (-1.83 eV) of the Pd active center. In conjunction with the intrinsic activity of the 1T-MoS2 monolayer for hydrogen evolution reaction, the Pd@1T-MoS2 SAC is a competitive and promising trifunctional electrocatalyst for sustainable energy conversion and storage systems.