The invention provides polyacrylonitrile mesoporous activated carbon fiber for a super capacitor electrode and a preparation method of the fiber. The preparation method comprises the steps that a pore-forming agent organic compound and/or a high polymer are/is added to a solvent, the mixture is mixed at the temperature of 50 DEG C to 80 DEG C by 3-24 hours, and a solution containing a pore-forming agent is obtained; polyacrylonitrile pulp is mixed with the solution containing the pore-forming agent, and a mixed spinning stock solution is obtained; spinning is performed with a wet spinning technology, and mixed protofilament is obtained; the mixed protofilament is soaked in a catalytic pore-expanding agent solution by 5-24 hours, the mixture is placed into an activating oven for pre-oxidation, activation and carbonization after being dried at the temperature of 80 DEG C to 120 DEG C, and the polyacrylonitrile mesoporous activated carbon fiber for the super capacitor electrode is obtained. The prepared polyacrylonitrile mesoporous activated carbon fiber has higher strength and is suitable for manufacturing the super capacitor electrode due to the appropriate mesoporous structure.