Electrochemical in situ hydrogen peroxide (H₂O₂) generation from a two-electron water oxidation reaction (2e-WOR) is a challenge, not only on catalyst selection but also on electrode making. Herein, the H₂O₂ electrocatalyst CaSnO₃ nanoparticles were prepared by low-cost glucose as an agent and characterized by X-ray diffraction (XRD), thermogravimetric and differential scanning calorimetry (TG-DSC), Fourier transform infrared spectra (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The active sites for the OH^– adsorption on the surface CaSnO₃ (121) was identified by density functional theory (DFT) calculation, and the corresponding reaction mechanism of H₂O₂ formation was proposed. The CaSnO₃ nanoparticles can be formed from 650 to 850 °C, and the particle sizes are in the range of 27.2–37.3 nm. The mechanism of catalyst formation is that species of Ca and Sn reacted with oxygen to generate CaO and SnO₂ during low-temperature calcination and CaSnO₃ generated during high-temperature calcination. The active sites are the coordination-unsaturated Sn ions, which easily adsorb the negative-charge OH^– from the solution, forming an OH* intermediate, and two adsorbed OH* can combine to generate a neutral H₂O₂ molecule. The H₂O₂ generation rate over CaSnO₃ was calcinated at 850 °C is 347.7 μmol·min^–1·g^–1 at 2.6 V versus Ag/AgCl under dark conditions. The work opens an in situ H₂O₂ generation route, direct water oxidation, with wide application prospects.