The application provides a method and device for measuring blood flow rate in a relaxing phase, a system and a storage medium. The method for measuring blood flow rate in relaxing phase comprises thefollowing steps of accessing a coronary artery two-dimensional contrastographic picture group of one or more body positions; extracting an interested blood vessel section from the coronary artery two-dimensional contrastographic picture group; extracting the center line of the blood vessel section; subtracting time for a contrast medium flowing through the blood vessel section in any two frames ofthe coronary artery two-dimensional contrastographic pictures, wherein the difference is delta t; subtracting the center lines of the section, wherein the difference is delta L; according to the ratio of the delta L to the delta t, solving the blood flow rate; and selecting the maximum value of the blood flow rate, so as to obtain the blood flow rate in the relaxing phase. Through the adoption ofthe method for measuring blood flow rate in relaxing phase disclosed by the invention, measuring by a non-invasive blood pressure instrument is not needed, and by the measuring method, the blood flowrate in the relaxing phase cannot be influenced by external factors including mood and the like, so that the accuracy of measuring the blood flow rate during the relaxing phase is improved.