Photocatalysts with excellent performance, low cost and innocuity are highly desired for environmental remediation. Zero-dimensional carbon dots with a size of 2-10 nm represent a class of promising co-catalysts due to their excellent photo-induced electron transfer, increased light absorption and thus boosted photocatalytic activity. Herein, carbon dots/high-crystalline g-C3N4 (CDs/H-CN) composites were successfully prepared via a facile calcination method for the degradation of tetracycline (TC) under visible light irradiation. Experimental results reveal that the CDs/H-CN-1 composite displays the optimal photocatalytic degradation of TC (86%, 120 min), and exhibits remarkable photostability (no decrease after 4 cycles of reaction within 480 min). According to the identification of intermediates by liquid chromatography-mass spectrometry (LC-MS) analysis, the degradation pathways of TC were proposed. In addition, the planting experiment of mung beans was performed to further confirm the biotoxicity of TC degraded products. This work underlines the importance of co-catalyst and presents a feasible protocol for the rational construction of H-CN-based photocatalysts for various photocatalytic applications.