In the present study, we combined brown TiO2 (denoted as TiO2-x) with Bi12O15Cl6 and BiOCl components via a one-pot hydrothermal route preceded by calcination at 500 ○C. The resultant novel photocatalyst was characterized by TEM, SEM, XRD, XPS, EDX, UV–vis DRS, FTIR, Raman, PL, photocurrent density, BET, and EIS analyses. The quantum dots (QDs)-sized TiO2-x/Bi12O15Cl6/BiOCl nanocomposite exhibited a significant photocatalytic ability upon visible light for the photodegradation of tetracycline (TC) with the rate constant of 379 × 10−4 min−1, which was 28.3 and 9.92 times as high as pristine TiO2 and TiO2-x, respectively. Moreover, compared with TiO2, the optimum photocatalyst presented boosted activity of 25.3 and 23.7 times in the degradations of rhodamine B and Fuchsine, and 23.0 folds in the reduction of Cr (IV) to Cr (III). The photoluminescence, electrochemical, and textural analyses showed that the boosted activity of ternary TiO2-x/Bi12O15Cl6/BiOCl photocatalyst is attributed to impressive separation and transfer of the photoinduced charges, and high surface area. The improved charge separation mechanism was devoted to developing tandem n-n heterojunctions amongst TiO2-x, Bi12O15Cl6, and BiOCl counterparts, as well as the construction of a QDs-sized photocatalyst. Moreover, the photocatalyst displayed considerable reusability in the TC degradation reaction. At last, the biocompatibility of the treated solution over the optimum TiO2-x/Bi12O15Cl6/BiOCl nanocomposite was proved by the growth of wheat seeds.