The invention relates to a method for co-combusting a biomass gasification gas through 600MW supercritical brown coal once-through boilers. A combustion system is formed by two 600MW supercritical brown coal once-through boilers and two biomass gasifiers; five gas combustors located between an over-fire air combustor and a third-layer pulverized coal combustor are arranged on rear walls of the twoboilers; the two gasifiers share one set of gas mother tube dividing into ten branch paths connected with ten gas combustors of the two boilers; air required for gas combustion is from secondary airducts of the boilers; the combustion system is on-line monitored and controlled through a distributed control system; and a 'one to one', or 'one to two', or 'two to one' operation mode can be selected according to different online loads provided by a power grid dispatching center, so that the biomass gasifiers can be coupled for generating electricity through different operation modes according to operation conditions and peak-pitching task requirements, and the economic efficiency and the safe reliability of unit operation are improved.