您还需要一个RewriteCond来检查主域：

RewriteEngine On

RewriteCond %{HTTP_HOST} ^(?:www\.)?example\.com$ [NC]
RewriteCond %{REQUEST_URI} !^/mainsite/ [NC]
RewriteRule ^(.*)$ /mainsite/$1 [L]

这样做，这条线帮助我解决问题。

# .htaccess main domain to subdirectory redirect
    # Do not change this line.
    RewriteEngine on
    # Change example.com to be your main domain.
    RewriteCond %{HTTP_HOST} ^(www.)?example.com$
    # Change 'subdirectory' to be the directory you will use for your main domain.
    RewriteCond %{REQUEST_URI} !^/subdirectory/
    # Don't change the following two lines.
    RewriteCond %{REQUEST_FILENAME} !-f
    RewriteCond %{REQUEST_FILENAME} !-d
    # Change 'subdirectory' to be the directory you will use for your main domain.
    RewriteRule ^(.*)$ /subdirectory/$1
    # Change example.com to be your main domain again.
    # Change 'subdirectory' to be the directory you will use for your main domain
    # followed by / then the main file for your site, index.php, index.html, etc.
    RewriteCond %{HTTP_HOST} ^(www.)?example.com$
    RewriteRule ^(/)?$ subdirectory/index.html [L]