把这段代码加到主题的 functions.php 文件里成功解决

//WordPress SSL 证书变绿色 update
add_filter('get_header', 'fanly_ssl');
function fanly_ssl(){
if( is_ssl() ){
function fanly_ssl_main ($content){
$siteurl = get_option('siteurl');
$upload_dir = wp_upload_dir();
$content = str_replace( 'http:'.strstr($siteurl, '//'), 'https:'.strstr($siteurl, '//'), $content);
$content = str_replace( 'http:'.strstr($upload_dir['baseurl'], '//'), 'https:'.strstr($upload_dir['baseurl'], '//'), $content);
return $content;
}
ob_start("fanly_ssl_main");
}
}