The invention discloses a bobbin surplus detecting method based on the machine vision technology. Residual bobbin side face (thickness) images are collected through an image sensor and transmitted to a computer. The computer processes the side face images through the image processing technology and judges the surplus, and when the set value is reached, a switching signal is sent to a winding and connecting unit. The bobbin surplus can be controlled to be very low, and waste caused by the original excessive surplus is avoided; the bobbin surplus detecting method can be suitable for scrolls with different diameters and bobbins with different thicknesses, and wide application space is achieved.