[junjiefu]

I cannot vouch for a complete absence of water, but no matter. One can get there either way. I'd be more inclined to have methanol attack the unoxidized sulfur to give two new sulfides. RSOH is noted to react with itself to form RS(=O)SR + water. So sulfur is still pretty nucleophilic. If no water, then form CH3Br. Give that a go.

Thank you so much for your reply！
Do you mean what I draw here?
How could it form RSOH? And one thing I don't quite understand is what means "unoxidized sulfur"?

[orgopete]

Re unoxidized sulfur, my poor reference to the sulfur you attacked with methanol.

You don't need to form RSOH. I referred to it in a reaction I knew. I suggest you might expect RSOMe to react similarly. Hint, bromide can remove a methyl group also and a loss of a proton is not needed.