The invention relates to a method for preparing nano silver through pyrenochaeta penicillium and belongs to the field of nano material preparation. According to the method, a pyrenochaeta penicillium chrysogenum soak solution mainly serves as a reaction solution to prepare the nano silver, operation is easy, no other reducing agent or additive needs to be added, the whole process is environmentally friendly, safe and non-toxic, reaction conditions are safe and controllable, the nano silver prepared through the method is spherical or approximately spherical, and the particle size is 5-60 nm. Due to the fact that reductase and other bioactivators in the pyrenochaeta penicillium chrysogenum soak solution participate in the process that silver nitrate is reduced into the nano silver and also wrap the surfaces of silver nano particles after the silver nano particles are formed, protein coats are formed, compared with nano silver and silver nitrate which are synthesized through a chemical method, the nano silver has unique biological characteristics and good biocompatibility, the nano silver can enter microbial cells more easily and disturb normal metabolism of microorganisms so that the microorganisms can be dead, thus, better bacteriostatic activity is achieved, application of the nano silver in the field of biomedicine is more advantageous, and the application prospects are broader.