The invention relates to a biogas residue organic fertilizer and a preparation method thereof. The biogas residue organic fertilizer uses biogas residue as major raw materials (the consumption is higher than 80 percent) to be mixed with humic acid, nitrogenous fertilizers, phosphate fertilizers, potash fertilizers and trace elements. By aiming at two problems that the biogas residue can not be dewatered and solidified easily and the mechanical dewatering has high energy consumption, the invention utilizes a shallow tank natural solidifying and air drying process, and the biogas residue carries out aerobic secondary fermentation, air drying and solidification in a shallow tank to meet the pelletization moisture and physical property requirements. The biogas residue organic fertilizer of the invention can meet the requirement of nutrient required by crop growth, and can also fertilize the soil and improve the yield.