This paper proposed a new method based on the use of the voltage ratio technique to measure the ac voltage at mV level. This method extends the voltage ranges from 2 mV to 200 mV with only one step by meaning of the cascaded design inductive voltage divider. Compared with micro-potentiometers method, this method greatly reduced transfer paths and decreased accumulation of measurement errors caused by the many steps transfer and improved the measurement accuracy of the ac voltage measurement at mV level.