The invention discloses a degradable nano water-retention mulching film and a preparation method of same. The mulching film is prepared from, by weight, 10-15 parts of nano CaCO3, 5-10 parts of nano TiO2, 10-20 parts of polyvinyl alcohol, 15-25 parts of polypropylene, 2-3 parts of polylactic acid, 12-27 parts of straw biomass charcoal, 10-20 parts of a MP3005 water-retention agent, 3-5 parts of an EM enlarging liquid, 15-26 parts of a compound plasticizer, 5-12 parts of a biodegradation promoter, 2.8-5 parts of a photo-initiator, and 1-5 parts of a starch adhesive. The preparation method is simple and low-cost and can improve the degradation speed of the mulching film. After degradation, residues of the mulching film can be absorbed and utilized by microorganisms in soil, so that not only is growth of crops improved effectively but also secondary pollution is avoided. When the components, such as the straw biomass charcoal, MP3005 water-retention agent and EM enlarging liquid, in the mulching film are degraded, structure and porosity of soil are improved, thereby reducing soil acidity and improving activity of microorganisms, and further improving water and fertilizer retention capability of soil.