The invention relates to a full-close type open type commutator. The commutator is characterized in that the commutator comprises a stator and a rotor which has relative rotation with the stator in the stator, wherein the inner surface of the stator and the outer surface of the rotor are in sealing fit by liquid; the stator is provided with a liquid inflow port and at least two liquid outflow ports; the rotor is provided with a connecting passage; when the rotor rotates, the liquid inflow port of the stator can be selectively communicated with one of the liquid outflow ports of the stator bythe connecting passage; and when the rotor is in the process of rotating and switching a flow passage, the liquid inflow port of the stator is always positioned in the caliber range of an inlet of the connecting passage. When the commutator is reversed, no phenomenon of sputtering is produced; and simultaneously, flow fields of the commutator before and after reversing are still symmetrical to ensure that two liquid outflow ports have consistent flow. As the inner surface of the stator and the outer surface of the rotor are in sealing fit by liquid, the commutator solves the problem of the overflow of steam when the flow of hot water is measured; and therefore, the commutator is suitable for a standard device of a heat meter.