The invention relates to a double machine measurement and control online main and standby switching method. A spacing layer measurement and control device of an entire transformer substation adopts the running mode of upper main standby and lower double main operation; in the aspect of switching logic, two measurement and control devices provide two hard contact outputs, namely, local locking andlocal main standby; when the hardware or communication of any one of the two measurement and control devices becomes abnormal and normal supervision of the transformer substation cannot be maintainednormally, a locking abnormity signal is output and a local locking signal relay is driven to output; simultaneously, a program of the measurement and control device realizes redundant monitoring of atransformer substation monitoring system according to the current main standby state of the measurement and control device. System redundancy of the transformer substation in a control direction and a monitoring direction can be realized by making little hardware improvement on the basis of dual measurement and control and the operating reliability of a transformer substation system is improved greatly.