An electrically-controlled large-capacity proportional valve includes, in the inside thereof, a depressurization chamber, which is connected to a guide hole channel, a sensor channel, and a discharge channel. The depressurization chamber is provided therein with a main diaphragm having a reversal prevention valve for driving a primary shaft and a secondary shaft to move. Through detection conducted in the sensor channel, when a pressure is low, the main diaphragm drives downward the primary shaft to push the secondary shaft, so that the secondary shaft causes a valve gate opening to carry out adjustment through two stages of displacement for controlling fluid flow rate after opening of the valve gate opening. At this moment, the primary-side pressure enters an output port of a valve seat the outlet port to increase a secondary-side pressure so as to accurately regulate an output of the secondary-side pressure.