Pneumatic valve-controlled micro-droplet ejection is a printing technique that has potential applications in many fields, especially in the field of bio-printing. The ejection is controlled by a solenoid valve being briefly turned on, so that high pressure gas enters the liquid reservoir, forming a gas pressure pulse, forcing the liquid out through a tiny nozzle to form a micro-droplet. For bio-printing applications, the bio-inks are typically non-standard. The difficulties are not only that the initial working parameters are difficult to set, but also the working conditions change over time in many cases. In order to maintain a stable single-drop ejection state, a machine vision based ejection monitoring was designed to obtain the number, positions and sizes of the droplets for each ejection, and a feedback control is realized by adjusting the conduction time of the solenoid valve or the gas pressure at the front end of the solenoid valve.