The invention relates to a method for fermenting and producing plasmin by use of cordyceps militaris liquid. The method comprises the following steps of: (1) inoculating the cordyceps militaris strain into the PD liquid fermentation medium to perform activation culture, and transferring to a seed fermentation medium to perform seed culture to obtain seed liquid; (2) inoculating the seed liquid into a liquid fermentation medium according to a volume ratio of 5-15%, and performing liquid fermentation culture for 20-30 hours; adding an expansion protein solution, and continuing culture for 120-168 hours; and separating and removing cordyceps militaris mycelium to obtain plasmin fermentation supernate; and (3) extracting cordyceps militaris extracellular plasmin from the plasmin fermentation supernate to obtain the cordyceps militaris plasmin. According to the invention, the plant expansion protein and the optimized fermentation method are applied to the liquid fermentation production of cordyceps militaris plasmin, and the liquid fermentation output of the cordyceps militaris plasmin is remarkably increased to 93270U per liter of fermentation broth, which is over 2.69 times that of the traditional fermentation production method; and the method has perfect industrial application prospect.