Abstract: Citric acid complexing method was adopted to prepare nano-crystalline La_0.4Sr_0.6MnO₃. The material was based on metal nitrate and citric acid, processed with dissolving, evaporation, drying, calcination and milling, etc. Techniques of XRD, SEM, and XPS were used to characterize its physical chemical properties and microscopic structure. The results showed that the catalyst was conformed to cubic phase perovskite, with spherical shape and good dispersity, and the proportion of each element met with the chemical formula basically. The photocatalytic performance of the nano-crystalline La_0.4Sr_0.6MnO₃ was explored with methyl orange (MO) under visible light. The photocatalytic activity was investigated by changing the experimental factors, such as pH of solution and oxidant. The results showed that the degradation performance of MO under the acidic condition (pH 1.4) was better than the alkaline condition, and the degradation of MO could reach more than 75% under the acidic condition. Meanwhile, nano-crystalline La_0.4Sr_0.6MnO₃ with oxidant showed better photocatalytic activity than that without oxidant. The adsorption and photo-oxidation properties of nano-crystalline La_0.4Sr_0.6MnO₃ were explored, and the results met with the Langmuir model and first order reaction kinetics model.