Abstract: Equivalent capacitances of thyristors of thyristor switch in the controlled metal oxide arrester （CMOA）,CT,changes with voltage,UT. When the voltage distribution of CMOA is calculated,such characteristic should be fully considered. Combining field-circuit combination method and variation rule of CT with UT,a calculation program was put forward. Firstly,electromagnetic-field numerical calculation models of controlled component,fixed component and resistance of thyristor were established and an equivalent capacitance network was set up. Then the relation between CT and UT,was solved by test and curve fitting. Secondly,an equivalent circuit of CMOA was founded,the relation between CT and UT was substituted to the circuit,then the function between the electrode voltage,UFi,and the CMOA voltage,Us could be calculated by iterations. Finally,the function between UFi and Us was substituted to the electromagnetic-field numerical calculation model,the variation rule of voltage distribution with Us was solved. The calculation program was verified by tests. Results indicated that it is effective to calculate the voltage distribution of CMOA.