Hi there @zhangke200377,
I took a look at our logs for your zone and can see that the intermittent 521 errors that have been served are mainly due to a network connectivity issue with Railgun. Can you reach out to your hosting provider or look through your origin server logs for any Railgun errors?
Here’s this for reference: https://developers.cloudflare.com/railgun/user-guide/set-up/potential-problems