Fish vaccine injection is an effective way to prevent fish diseases. At present, the injection of Ctenopharyngodon idellus vaccine is entirely by hand. Aiming at the low efficiency of manual injection and the poor precision of injection caused by the experience and hand feeling of the inject location, a scheme of replacing manual injection with mechanical automatic injection was proposed. A Ctenopharyngodon idellus vaccine injection machine with high degree of automation was developed, and object morphology was used to analyze the characteristics of Ctenopharyngodon idellus, so as to realize the function of turning the head and tail of the fish; the grating measuring sensor was used to calculate the length of Ctenopharyngodon idellus; the chromaticity sensor was used to distinguish the fish abdomen and fin; the photoelectric sensor was used to separate different channels to achieve multi-channel injection of Ctenopharyngodon idellus vaccine; the stepper motor was used to achieve accurate fish picking, as well as the automatic adjustment of injection positions of fry of different lengths; the discriminating device of the injection machine controlled by Siemens PLC was coordinated with the power execution unit to realize precise injection. The performance test results show that when the belt transmission speed was 2 m/s, 100-120 mm fry could be automatically sorted and injected continuously and rapidly. The injection efficiency was about 900 fry/h, and the fry that did not meet the conditions could be returned to the pool. Three weeks after automatic injection of normal saline, the survival rate of the tested fry was 98.8 %. The research shows that the control system is reliable and efficient, which lays a foundation for the research and application of China fish vaccine injection.