Hi，after i buy the ssl certificate and change the setting i cannot load the site

Hey zqmaqma,

Try to add this code to your .htaccess to redirect all requests to the new https url:

RewriteEngine On
RewriteCond %{HTTPS} !=on
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

If this doesn’t help try to download and install the php script from here: https://interconnectit.com/products/search-and-replace-for-wordpress-databases/

Use ftp to upload it to the main wordpress directory (where the wp-content, wp-admin, etc. folders are located). Then replace all occurrences of the old http:// url with your new https:// url.

Best regards,
Dude