The invention relates to the field of papermaking and fertilizer production, in particular to a method for preparing organic-inorganic mixed fertilizer from black liquor from KOH and NaOH mixture alkaline pulping for straw. The method includes: charging the black liquor from mixed alkaline boiling to an evaporation section, evaporatively concentrating the mixed black liquor into concentrated black liquor, and extracting organics and fertilizer elements such as nitrogen, phosphorus and potassium in the concentrated black liquor to produce the high-activity organic-inorganic mixed fertilizer. The organic-inorganic mixed fertilizer has multiple effects, high efficiency, complete functionality and better nutrition and is more suitable for growing plants; the organics may improve the physical, chemical and biological features of soil, maturate the soil and improve soil fertility and may also provide inorganic nutrients such as nitrogen, phosphorus and potassium.