The present disclosure provides a method for preparing a fermented soybean paste and a fermented soybean paste prepared thereby, the method comprising: a mixing step for mixing a Traditional Korean fermented soybean paste and a modified Korean fermented soybean paste to prepare a mixed fermented soybean paste; a first aging step for primarily aging the mixed fermented soybean paste at room temperature; and a second aging step for secondarily aging mixed fermented soybean paste, which has gone through the primary aging at low temperature. According to the method for preparing a fermented soybean paste and a fermented soybean paste prepared thereby, of the present disclosure, a distinctive deep and pleasant flavor of the Traditional Korean fermented soybean paste due to the combinative fermentation is maintained by mixing the traditional Korean fermented soybean paste and the modified Korean fermented soybean paste, and the protein degradation rate is increased using the high protease activity of the modified fermented soybean paste, thereby shortening the aging time and increasing the savory taste. In addition, according to the method for preparing a fermented soybean paste of the present disclosure, the mixed fermented soybean paste having gone through primary aging is secondarily aged at a low temperature, thereby preventing browning during the process, so that a fermented soybean paste with high preference can be produced, and the preparation time is relatively short while a pleasant flavor of the Traditional Korean fermented soybean paste is maintained, so that a fermented soybean paste with excellent quality can be mass-produced.