An IO design method supporting impedance matching is proposed in this paper. The IO supports multi-level standard.An automatic impedance matching module which is easy to implement is designed. Firstly, it compares the nominal resistance provided by users with the internal IO BUF impedance. After the impedance adjustment in a certain period, the internal BUF impedance can match with the nominal impedance provided by users, so as to realize the function of multi-voltage driving and automatic impedance matching. The simulation results show that the circuit can work above the frequency of 100MHz, and rapidly implement the automatic impedance matching.