A novel ternary SnO/g-CN/diatomite (SCN/DE) nanocomposite was rationally designed and successfully synthesized via a two-step method with in-situ polymerization and self-assembling. Under visible light illumination, the resulting SCN/DE composite exhibited superior photocatalytic performance and good reusability for the photoreduction of Cr(VI) to Cr(III) in the presence of citric acid, the apparent rate constant of SCN/DE composite was up to around 22.68 times, 13.53 times and 8.65 times as much as those of g-CN (CN), g-CN/diatomite (CN/DE) and SnO/g-CN (SCN) composites, respectively. The citric acid is a co-catalyst (chelating agent) rather than a reactant during the reactive process. Besides, the intimate interface contact and ternary heterogeneous structure were established among the SnO, g-CN and diatomite. The induced positive charged surface of diatomite should be the key factor in enhancing photoactivity of the resultant SCN/DE composite, which significantly accelerated the charge separation of photogenerated electron-hole pairs as well as improved the adsorption performance towards Cr (VI). In particular, a possible reduction pathway of Cr(VI) to Cr(III) by SCN/DE composite with the assistance of citric acid was first investigated and proposed. This work provides a novel strategy for synthesizing highly efficient mineral-based photocatalysts with great promising application foreground for Cr(VI)-containing wastewater treatment.