Actually, I want to know it at a particular time period in whole time process (such as, 0--500ms in -500--1000ms). If I want to leave time out and do a frequency analysis, is the step similar with time-frequency?
If you want an average value for all the time definition: use the PSD process instead of a time-frequency approach.
If you want the average of a shorter time window, compute the TF of the whole epoch (-500,1000ms), then compute the average over the period of interest (eg. 0-500ms) with the process "Average > Average time" or "Extract > Extract values" (with the averaging option selected).
Maybe I should calculate it based on the results of the time-frequency？Just sum up to the total power and divide the power of one band by that total power?
This is exactly what the process "Standardize > Spectrum normalization" with the option "Relative power" does:
If you want to apply another measure, you can do it either with the process "Pre-process > Run Matlab command" or by exporting the files to Matlab, processing them the way you want, and re-importing the modified values:
https://neuroimage.usc.edu/brainstorm/Tutorials/Scripting#Custom_processing