The UV/sulfite system is a promising source of •SO₄⁻ and/or •OH, but its application is largely limited by the use of UV light due to its high cost and high energy consumption. Graphite carbon nitride (g-C₃N₅), as a new photocatalytic material, has better visible light absorption capacity and narrower band gap than g-C₃N₄, which is expected to activate sulfite under visible light to solve this problem. Herein, a novel FeS₂/CN heterojunction material based on g-C₃N₅ was constructed by hydrothermal in-situ synthesis method and successfully activated sulfite, which was confirmed by tetracycline degradation experiments in water. Under optimized conditions, the degradation rate of TC in 1 h reached 96%. The experimental results revealed that the FeS₂/CN heterostructure enhances the absorption of visible light and inhibits the recombination of carriers, enabling more electrons and holes to be utilized. Holes play a major role in the degradation reaction, promote the sulfite chain reaction, and effectively regulate the cycle of Fe²⁺ and Fe³⁺ in the solution. Iron ion leaching is negligible and the degradation reaction remains stable at pH 5–9.