The metrological performance of flow meter should be tested under the condition of ambient temperature and humidity changes according to appendix A type evaluation outline of JJG 1033-2007 "Electromagnetic Flowmeter" verification procedures. In order to meet the requirements of the type of evaluation outline, an "environmental test on-line detection device based on flow metrological instrument" is developed. The pipe and the meter can be directly connected to this device. The metrological performance of the meter can be assessed and the environment’s temperature and humidity can be controlled at the same time. Experts have identified that the device can fully meet the requirement that metrological performance of flow meter can be tested under the condition of ambient temperature and humidity changes.