Gd3+-doped Bi4O5I2 with a flower-like structure has been successfully prepared for the first time using a one-step hydrothermal method. The effect of different doping amounts on the photodegradation properties has been investigated. The total removal of ciprofloxacin (CIP) by 3 % Gd–Bi4O5I2 (molar atomic ratio of Gd to Bi = 0.03:1) under visible light irradiation was 94.50 % (within 120 min). The reaction rate constant was 6.3 times higher than that of pure Bi4O5I2. In complex water containing humic acid (HA) and various anions, 3 % Gd–Bi4O5I2 maintained a high total removal rate for CIP. UV–Vis diffuse reflectance spectroscopy and electrochemical tests have shown that doping Gd3+ not only extends the photoresponse range, but also reduces the photonic band gap width. Gd3+ ions act as electron traps to trap excited electrons and promote the separation and migration efficiency of photogenerated electron-hole pairs, improving the performance of Gd–Bi4O5I2 composites for the photocatalytic degradation of CIP.