China Bank's flash payment function is a "small amount/signature-free" function, which can be closed through Bank of China's mobile banking. The specific operation steps are as follows:

1. After opening the mobile banking of Bank of China, enter the bank account number and password in the login interface, and click "Login" to enter the personal page of mobile banking.

2. Log in to the home page of mobile banking and click the "Account Management" function button to set it.

3. Click the "Electronic/Cash" function button on the transfer management details page.

4. Enter the electronic cash management page and click the ellipsis in the upper right corner.

5. Enter more pages to see a series of settings about account number and card, and click "Transaction Limit Settings" to set them.

6. On the transfer limit setting page, it shows that the "small amount/signature confidentiality" item has been opened, and click Enter.

7. Click the switch to the right of "signed small/confidential", and the button will be displayed in green when it is turned on.

8. Click "OK" in the pop-up closing confirmation dialog box.

9. The function of "small/signature-free confidentiality" was closed successfully, and the button on the right side was grayed out.

In this way, the flash payment function of China Bank is closed.