However, it only triggered automatically in Gnome activity and the application menu, nowhere else. See this bug . Although it is marked as fixed, but my laptop still have this problem.
For those who have the same problem, you can install the previous Ubuntu 16.04 default screen keyboard instrument onboard as a solution in Ubuntu 18.04. The most significant feature of this software is that the keyboard can be mapped to the screen, then we can use the mouse for input on the keyboard, the software provides a variety of keyboard types to choose from.
Open the Ubuntu software, and search for and install Onboard Onboard settings.
Once installed, start the utility from Gnome application menu.
In Onboard settings, set the automatic display, layout, themes, OK, you have to try it.