Hi Chen Ming,
Thank you for your question. Please refer to this tutorial page on our website that explains in more details the source reconstruction methods available in Brainstorm: https://neuroimage.usc.edu/brainstorm/Tutorials/SourceEstimation
You will find at the end of this page paper references for each major method implemented in Brainstorm. The one you are refering to is the minimum norm, and the following paper goes into more details about the mathematical formulas behind it: https://neuroimage.usc.edu/paperspdf/BailletMosherLeahy_IEEESPMAG_Nov2001.pdf
If you have more questions about Brainstorm, be sure to post them on our very active user forums: https://neuroimage.usc.edu/forums/
I hope this helps!
Martin