The invention provides a method and device for determining inlet air quantities and an automatic control system, belonging to the technical field of combustor operation air quantity control. The method comprises the following steps: determining primary inlet air quantity of a coal pulverizer according the coal supply quantity of the coal pulverizer and the air/coal ratio of the in-furnace coal quality; and determining secondary air box inlet air quantity at the air box inlet provided with a combustor according to the analytical data of the in-furnace coal quality, the coal supply quantity of the coal pulverizer and the primary inlet air quantity. The method can be used for quickly determining the optimal primary inlet air quantity and secondary air box air inlet quantity of the low-NOx cyclone combustor according to the coal supply quantity of the coal pulverizer, the air/coal ratio of the in-furnace coal quality and the analytical data of the in-furnace coal quality, implements automatic control on the primary and secondary air quantity ratio of the low-NOx cyclone combustor in the boiler operation process, and can keep the hearth outlet NOx concentration on a low level on the premise of ensuring the high-efficiency combustion.