The invention discloses method of producing liquid organic fertilizer by organic belonging to solid wastes disposal field, including: co-treating the residual sludge with 98 percent moisture content by ultrasound and sodium hydroxide to make the organic matters dissolve out; getting concentrated sludge and supernatant fluid after centrifugation; dehydrating the concentrated sludge after neutralization in a routine method; sand leaching and ultrafiltration treating the supernatant fluid twice after centrifugation to get a concentrated liquid containing macromolecule organic matter such as humic acid and the filtrate containing low molecular organic matter; the filtrate entering into the sewage treating plant for a primary treatment. Disposed dehydrated sludge cake has 70-80 percent moisture content and a reduction of the content of organic matter of 30-50%. The concentrated liquid by ultrafiltration is mainly composed of macromolecule organic matter and with a low content of heavy metal, and can be an organic liquid fertilizer, and complies the rule of using green food fertilizer. The invention is suitable for the sludge with a high organic matter content such as residual sludge and digested sludge.