Hi, @Ying Wu
You can use it for production.
Please read the following Q & A.
Can the Azure free account be used for production or only for development?
The Azure free account provides access to all Azure services and does not block customers from building their ideas into production. The Azure free account includes certain types of specific services—and certain amounts of those services—for free. To enable your production scenarios, you may need to use resources beyond the free amounts. If you choose to move to pay as you go, you’ll be billed for those additional resources at pay-as-you-go rates.
Also, be aware of free subscription limits and upgrades.