The invention provides an image collecting device, system and method for tree DBH measurement. The image collecting device comprises a handheld device, a shooting device and a light source emission device; the shooting device and the light source emission device are arranged on an image collection bearer of the handheld device; and when the shooting device collects a DBH picture of a tree to be measured, the light source emission device emits horizontal light to the tree to be measured, and the horizontal light is vertical to the growth extension direction of the tree to be measured. The system comprises a processor and the image collecting device. The method comprises that the DBH picture, with the horizontal light, of the tree to be measured is collected, the number of pixel points of the horizontal light in the DBH picture is obtained, and further the DBH of the tree to be measured is obtained. According to embodiments of the invention, the entity structure is simple, the device is convenient to carry, the DBH picture, with the horizontal light, of the tree to be measured can be obtained rapidly, and the DBH measuring efficiency and precision of the tree are further improved.