The insulating oil micro-water tester uses the classic theory-Karl Fischer microcoulomb electric quantity method; the number of water molecules reacted according to the law of electrolysis is proportional to the number of charges, and the number of charges (coulombs) detected by the instrument is automatically converted into the corresponding water molecules. Therefore, this method has extremely high test accuracy and low test cost, and has the irreplaceable advantages of other test methods; it can reliably determine trace moisture in liquid, gas, and solid samples. It has high sensitivity, high precision, high reproducibility, low power consumption and energy saving design. It can be used for portable measurement with built-in battery. It is widely used in petroleum, chemical, electric power, pharmaceutical, commodity inspection, scientific research, environmental protection and other fields.