The work aims to quantitatively evaluate the aging state of the outer coating for storage tank through evaluation criteria for coating aging and analytic hierarchy process. By studying and comparing the standard of coating aging rating method and introducing the analytic hierarchy process, the weight of a single aging performance index respected to the total index was calculated to establish the mathematical model of coating aging to quantitatively evaluate the coating state. The weight of rust, flaking, cracking, blistering and powdering calculated by analytic hierarchy process was 0.0508, 0.0292, 0.107, 0.061 and 0.032, respectively, among which rust had the highest weight. The anti-corrosion aging performance of an oil depot in Nanning was tested and evaluated. The test results showed that the aging state scores of only 3 layers among 8 layers of ring plates on the tank wall were between 45 and 50, and the aging state scores of the other 5 layers of ring plates, tank top and accessories were all less than 44. Considering the aging situation and economic benefits, it was recommended to re-coat the whole outer coating of the tank. The current evaluation standard for coating aging only has a stipulation scales on the evaluation method and rating of coating aging and has some shortcomings such as insufficient quantization and missing maintenance suggestions. By introducing the method of analytic hierarchy process (AHP), the aging state of outer coating can be quantified. Based on the quantitative evaluation results, appropriate maintenance suggestions can be given directly.