502 bad gateway with ASTPP

I am using nginx with 1.12.2 to access ASTPP, but the web browser shows 502 bad gateway. After invetigation, it may relate to nginx buffer config problem.

/etc/nginx/conf.d/cent_astpp.conf

server {
listen 8089 default_server;
listen [::]:8089 default_server;

root /var/www/html/astpp;
client_max_body_size 8M;

1. Add index.php to the list if you are using PHP
   index index.php index.html index.htm index.nginx-debian.html;

server_name _;
access_log /var/log/nginx/astpp_access.log;
error_log /var/log/nginx/astpp_error.log;

location /

{ try_files $uri $uri/ /index.php; }

1. pass the PHP scripts to FastCGI server listening on 127.0.0.1:9000
   #
   location ~ \.php$ { fastcgi_connect_timeout 300; fastcgi_send_timeout 300; fastcgi_read_timeout 300; fastcgi_split_path_info ^(.+\.php)(/.+)$; # fastcgi_pass unix:/var/run/php5-fpm.sock; fastcgi_pass 127.0.0.1:9000; fastcgi_index index.php; fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name; include fastcgi_params; fastcgi_buffer_size 128k; fastcgi_buffers 256 4k; fastcgi_busy_buffers_size 256k; fastcgi_temp_file_write_size 256k; }

   }