The AEC-Ⅰ automatic exhaust control instrument with a vacuum pump can be used to exclude rudimental radon from water radon scintillation chamber. The device has the characteristics of stable performance, simple operation and in scintillation chamber it can be automatically and circularly operated in order of "exhaust-vacuum-exhaust again-vacuum again". It can also adjust the cycle time of "exhaust-vacuum" status according to the need and improve the exhaust efficiency. After exhausting, the background of scintillation chamber can achieve the requirement of "standard", and we can visualize the exhaust time on the device panel. During the trial operation of the device, many factors are analyzed, such as the stability of device, the influence of vacuum time and exhaust time on exhaust effect, the influence of continuous service time on the exhaust effect, the contrast with results of manual exhaust and so on. The results show that:AEC-Ⅰ automatic exhaust control instrument has high precision on control, stable performance, and its exhaust effect in scintillation chamber can meet the requirements of specification, so it can be used as the water radon observation auxiliary device in the exhaust of scintillation chamber.