what happen if a high voltage connects to pc-based oscilloscope's ground?
You must assume that at some point high voltage will be placed on your "scope module" incorrectly and design it so the user is safe according to for instance Low Voltage Directive (LVD) 2014/35/EU. You will never get certification unless stringent rules are followed depending on where your market will be. As for connecting to a PC you will want to use optical isolation to relay the data back and forth with optimally no electrical connection between the two at all. Depending on the marketing area you will want to review government mandated requirements. This can get real Hairy my friend!