The invention relates to a urea tank diagnostic system. The system comprises a post-treatment system controller ACU, an electric control unit E-box and computer, a urea tank diagnostic control systemis installed on the computer, the urea tank diagnostic control system comprises a safe access unit, a super-control test function unit, a fault clearing function unit, a real-time monitoring unit, a controller signal timeout checking function unit, an analog controller function unit, a data analysis function unit, the real-time monitoring unit comprises a controller static information reading module, an ACU signal reading module, an ECU signal reading module, an E-box signal reading module and an instrument signal reading module. The method is reasonable in concept, the method is stable in operation and use, good in diagnosis effect, high in efficiency and convenient to sell, the current state of the post-processing system can be read by the after-sales person, the current fault can be removed, can execute an over-control test and provide a diagnosis tool for after-sales and after-sale work.