1. Enter xrandr on the Raspberry Pi terminal to check the HDMI-ID where the main screen is located (mostly HDMI-1);
2. Enter xinput in the Raspberry Pi terminal to check the touch ID where the main screen is located (there are 2 touch IDs, if you don't know which one is the main screen, you can try both);
3. Run the command: xinput map-to-output <touch ID> <HDMI-ID>
(After matching the ID value, run the command to specify the touch to the main screen, such as: xinput map-to-output 7 HDMI-1 )
4. Since the command needs to be re-entered every time the boot is turned on, this command can be added to the boot auto-start:
sudo nano /etc/xdg/lxsession/LXDE-pi/autostart
Add: xinput map-to-output 7 HDMI-1 (remember to change to your own corresponding ID), then restart.