The invention aims to provide a gas-water balance dynamical control method for a continuous-casting machine, and aims to realize real-time response to secondary cooling water pressure change due to various causes of height difference adjustment, dynamic water amount adjustment and the like through dynamical adjustment of the pressure of secondary cooling compressed air and keep gas water balance at an optimal atomizing effect required value. According to a technological means, the method comprises the following steps: determining a gas water balance state of a nozzle required for achieving an optimal atomizing cooling effect through cold-hot state experiment data of the nozzle during model selection of the nozzle so as to set a set curve for air pressure-water amount in each cooling segment; then, arranging a pressure adjusting valve and a pressure transmitter in a compressed air inlet pipeline of each cooling branch; dynamically controlling the secondary cooling compressed air inlet pressure of each branch by adopting a PID (Proportion, Integral and differential) adjusting way and taking a current water amount of each branch as an independent variable according to the determined set curve of the air pressure-water amount of the nozzle so as to always keep the gas water balance in an optimal state.