Taking the once-through boiler equipped for ultra-supercritical 1 000 MW unit manufactured from Dong fang Boiler Works as the object of study and starting from the quality and energy conservation equation

a non-linear dynamic model of lumped parameters for the intermediary point temperature control has been established.By using a small deviation method to seek solution of the equation

the transfer function of the intermediary point temperature variation related to feed-water and feed-coal change has been obtained in the case of the once-through boiler being changed in the vicinity of one stable operating condition.