Finally, I found the root causes:

1. I use the command Get-WmiObject -Namespace "root\acim" -ClassName "HwSetting" | Get-Member to find that the registered method is SetHw(System.String name, System.String value), but it doesn't match my setting of SetHw() which does not have any arguments. So I re-compiled mof by mofcomp.exe.

2. We must implement GetObjectAsync() when we use a command like this (Get-WmiObject -Namespace "root\acim" -ClassName "MyHwSetting").MySetHw()