The present invention relates to a blood cell-free DNA-based method for predicting prognosis of breast cancer treatment and, more particularly, to a cell-free DNA-based method for predicting prognosis of breast cancer treatment, the method comprising a step of extracting cell-free DNA (cfDNA) from a biological sample before anticancer treatment, acquiring sequence information, then obtaining an I-score by using normalization correction and regression analysis of chromosomal regions, and analyzing the I-score and image information of the breast together after the anticancer treatment. A method for predicting prognosis of breast cancer, according to the present invention, uses next generation sequencing (NGS) so as to increase the accuracy of predicting the prognosis of a breast cancer patient and also increase the accuracy of prognosis prediction based on a very low concentration cell-free DNA of which detection has been difficult, thereby increasing the commercial utilization thereof. Therefore, the method of the present invention is useful in determining the prognosis of a breast cancer patient.