Hi theerapat259,
There are few tips you can implement in order to maximize the battery life on your Apple Watch.
Check out more information here: Batteries - Maximizing Performance - Apple
Since noticing this, have both your Apple Watch and your companion been restarted? Restarting can help resolve many unexpected issue we may face with our devices.
To restart your Apple Watch:
- Press and hold the side button until you see the power button
- Tap the power button.
- Drag the Power Off slider.
- After your watch turns off, press and hold the side button again until you see the Apple logo.
Restart your Apple Watch - Apple Support
You can follow the steps here: Restart your iPhone - Apple Support
You should also confirm that all software updates have been installed for both your Apple Watch and your iPhone.
Check out the following resources:
Update your iPhone or iPad - Apple Support
Update your Apple Watch - Apple Support
If this continues, you may want to reach out to Apple Support for further assistance. Depending on your region, you may be able to reach them here: Get Support.
Hope that helps!