The work aims to develop an online detection system for printing dot gain for the purpose of quickly and accurately detecting the print quality in the high speed printing and giving data evaluations. The printing dot images were obtained through the image acquisition system firstly. Then, the dot images were identified by means of the machine vision technology and Matlab advanced programming language. By comparing the standard dot images with the actual dot images, the effective recognition algorithm was used to calculate the gains of the three shapes of printing dots at 50%. The recognition algorithm has strong applicability and it can be applied in the online identification of gains of all shapes of dots in the color dot printing.