Hi Yi_Zheng,
A little more information might give us some ideas.
1.What is your XIAO? nRF52840 or ESP32C3 or ?
2.What is your battery? Is it 3.7V?
3.Can the battery be charged with another charger?
4.What is the voltage of the battery before connecting it to XIAO?
5.What is the voltage at the battery terminal when connected to XIAO and charging via USB?
The results I have is 2.38V and ~15mA. is it normal?
How did you measure the current 15 mA?
What measuring instrument did you use?
Where did you connect the probe?