The invention provides a method and a device for measuring the size and the distribution of middle and small liquid drops in fluid. The device comprises a measuring kit and a gas control part, wherein the measuring kit comprises a kit body with an opening and a closing door for closing the opening, and a detection piece bracket which is used for detachably loading a detection piece is arranged inside the kit body; the gas control part comprises a high-pressure gas source, a gas transmission pipe, an electromagnetic directional valve and a gas cylinder; the gas cylinder is provided with a gas cylinder shaft; the gas cylinder shaft is connected with the closing door of the measuring kit, wherein the gas control part takes gas from the high-pressure gas source as power and controls the closing door of the measuring kit to open and close by controlling the movement of the gas cylinder shaft. The method comprises the following steps: putting the device into measuring fluid atmosphere to cause small liquid drops in the fluid to impact the detection piece for leaving out a mark; and analyzing the mark on the detection piece to obtain the size and the distribution of the granular diameter of liquid drops. The device has simple structure and low cost, is flexible and convenient in operation and applicable to operations in most environments.