1. A method for calculating a physiological index, applicable to an electronic device, the method comprising: dividing a physiological data sequence into a plurality of windows, wherein each window comprises a data segment of the physiological data sequence; analyzing the data segment in each window to obtain metadata that represents data characteristics of the data segment; updating the metadata comprising the data characteristics of all data segments in the windows up to a previous window by using the metadata corresponding to one of the windows to obtain the metadata comprising the data characteristics of all data segments in the windows up to a current window; and calculating a physiological index by using the updated metadata, wherein the step of analyzing the data segment in each window to obtain the metadata that represents the data characteristics of the data segment comprises: using a plurality of scales to perform a coarse-graining procedure on the data segment in each window to obtain a data sequence under the scales and using the data sequence as metadata that represents the characteristics of the data segment, and wherein the step of updating the metadata comprising the data characteristics of all data segments in the windows up to the previous window by using the metadata corresponding to one of the windows to obtain the metadata comprising the data characteristics of all data segments in the windows up to the current window comprises: as for the data sequence under each scale of each window, calculating a distribution statistics corresponding to one of the windows by using a mathematical statistics analysis method to serve as metadata corresponding to the window; and updating the metadata by using a distribution statistics corresponding to another windows in sequence to obtain metadata that represents a distribution statistics up to the current window.