We propose a novel concept named atom-valley locking (AVL) base on two-dimensional (2D) non-magnetic pentagonal CdSe2 monolayer. Here, “locking” refers to an atom-dependent valley distribution. Protected by the four-fold rotational reflection symmetry (S4) solely, two sets of valley electrons can be distinguished both in real space and momentum space. Next, we construct a ternary CdSeTe monolayer to introduce an intrinsic electric field (EF) into the structure without destroying the AVL feature. In momentum space, a spontaneous polarization of 187 meV does not involve time-reversal symmetry can be observed. In real space, each set of the valley electrons can be associated with a distinct band-edge alignment (BEA). Then, we design a selective switch for BEAs via strain engineering, offering precise control of the valley-selective and face-specific hydrogen evolution reaction (HER) site. These results suggest that valleytronics hold great potentials in the field of photocatalytic water splitting.