The invention discloses a method for fixing a fish brain tissue specimen through systemic heart perfusion. The method is mainly characterized in that after fish anesthesia, thoracotomy is performed on the periphery of a heart surface projection area of the fish, ventral aorta intubation is performed through the ventricle, common cardinal vein intubation is performed through the atrium, ligation with an operation thread is used for fixing, and an out flow tube is connected with a graduated vessel for measuring liquid out-flowing quantity; a constant-speed perfusion pump is used for perfusing normal saline through a perfusion tube, and a 10% formalin solution is adopted for reperfusion; when perfusion is started, a line is marked on the skull instantly and craniotomy is performed, and the perfusion time is then decided through combination of the perfusion quantity and the out-flowing quantity when the color of the brain tissue changes white; the brain is picked and placed in the 10% formalin solution to be fixed for 48 h after perfusion is stopped, and the brain tissue specimen with complete structure, normal form and good state is obtained. By means of the method, the problems of insufficient fixing, non-comprehensive fixing area, tissue autolysis, cell deformation and the like of the fish brain tissue can be solved effectively, the brain tissue specimen with more complete structure and more ideal state is obtained easily, and the natural form is retained better.