As I knew, The trial is free and you will not be billed. When your trial ends, your account will be paused and you'll have the option to upgrade to a paid account.
But I have enabled billing in order to use Compute Engine's VM instance in my free trial account, will it bill from my credit card?
Which mean when free trial end, the billing will auto disable, it won't charge my credit card right?
When you create a project under free trial you get 300$ credit for 60 days. You need to provide a valid credit card in order to setup your account for free trial. During the free trial your monthly cost for the cloud services will be covered by the credit amount. In case if your cost is more than the credit amount your account will be suspended. As such, you will not be charged on your credit card. In order to unsuspend your project you need to upgrade your account.
You will have the option to upgrade your account at anytime during the trial period. If you don't upgrade your account in the first 60 days your project will be suspended and you need to upgrade your account manually in order to keep using the resources in your project.