The invention discloses a lachnum and a method for preparing a melanin by liquid fermentation thereof, and is characterized in that seed liquid which is obtained by the shaking culture after the Lachnum with a preservation number of CCTCC No: M 209193 is activated is inoculated into a fermentation medium to conduct liquid fermentation according to 4-8 percent of the fermentation medium in volume; after the fermentation liquid is extracted with alkaline solution, a precipitation agent is used for precipitation; by the acid hydrolysis of hydrochloric acid, a crude melanin is obtained from a crude extract; then the crude melanin is re-dissolved in ammonia solution; and finally a pure melanin is obtained by using oil ether, chloroform and ethyl acetate for extraction in sequence. Compared with the method which adopts a basic fermentation medium, by adopting the modified fermentation medium and taking the hydrochloric acid methanol solution as a precipitating agent, the yield of the melanin is increased to 3.856g/L from 1.471g/L, the fermentation period is shortened to 8 days from 12 days, and the purity of the crude melanin extract is high, thereby being conducive to the purification and reducing the damage to the structure thereof in the purification process.