The photocatalytic, structural and transport properties of the newly synthesized sulfide oxide CaZnSO and SrZnSO compounds are comprehensively investigated by means of first and second-principles calculation in order to explain the semiconductor's 'photo-excitatiod' state mechanism in CaZnSO and SrZnSO. At the same time, the influence of the substitution of Ca2+ by Sr2+ on the structural properties and, hence, on the photo catalytic properties, are investigated. The optical conductivity and the absorption level exhibit an obvious enhancement from the ultraviolet to the visible light region when we move from Ca to Sr. This shows that the absorption edge moves from lambda = 387.4 -> lambda = 442.7 nm, which corresponds to the direct optical band gap of 3.2 eV -> 2.8 eV, which is well matched with the solar spectrum and the sufficient negative conduction band potential for reduction of H+/H-2. The calculated electronic band structure and the angular momentum character of various structures confirm that CaZnSO and SrZnSO possess a direct fundamental energy band gap of about 3.7 eV (CaZnSO) -> 3.1 eV (SrZnSO), and the electronic charge distribution reveals a clear map of the electronic charge transfer and the chemical bonding. Furthermore, the carrier concentration (n) as a function of chemical potential at three constant temperatures (T) and n as a function of T at fixed chemical potential were calculated. It was found that n increases exponentially with increasing T and reveals that the CaZnSO and SrZnSO are p-type semiconductors. Based on these results, one can conclude that CaZnSO and SrZnSO satisfied all requirements to be an efficient photocatalyst. This will greatly improve the search efficiency and greatly help experiments to save resources in the exploration of new photocatalysts with good photocatalytic performance.