Hi Zhaoxin,
I am not sure what kind of mean you exactly want to get, but here are a couple of choices:
(1) You can use 3dTstat with its option -mean to get the mean for each voxel: [
afni.nimh.nih.gov]
(2) 3dMean: [
afni.nimh.nih.gov]
(3) 3dcalc: [
afni.nimh.nih.gov]
Let me know if I can be of any further help.
Cheers,
Gang