A novel 1D/2D step-scheme Bi2O3/g-C3N4 was prepared using a simple reflux method. Bi2O3 photocatalysts showed lower photocatalytic activity for the degradation of tetracycline hydrochloride under visible light irradiation. After compositing with g-C3N4, the photocatalytic activity of Bi2O3 was enhanced obviously. The enhanced photocatalytic activity of the Bi2O3/g-C3N4 photocatalysts could be attributed to the high separation efficiency of carriers generated by the Bi2O3/g-C3N4 photocatalyst due to the formation of a step-scheme heterojunction, which inhibited the recombination of photogenerated electrons and holes. In order to further enhance the degradation efficiency of tetracycline hydrochloride, Bi2O3/g-C3N4 was used to activate peroxymonosulfate under visible-light irradiation. The influences of peroxymonosulfate dosage, pH value and tetracycline hydrochloride concentration on activating peroxymonosulfate to degrade tetracycline hydrochloride were investigated in detail. The mechanism of Bi2O3/g-C3N4 activating peroxymonosulfate was proved by radical quenching experiments and electron paramagnetic resonance analysis, which proved that the sulfate radical and hole dominated the degradation of tetracycline hydrochloride. The possible vulnerable sites and pathways of tetracycline hydrochloride were predicted via DFT calculations based on Fukui function and UPLC-MS. Toxicity Estimation Software predicts that the degradation processes of tetracycline hydrochloride could gradually reduce toxicity. This study could provide an efficient and green method for the subsequent treatment of antibiotic wastewater.