Occupy as much as 600mb Memory in QNAP?

I found many of them are httpd 6 threads, totally occupied around 600mb. But my NAS max. 1gb for memory. Is that normal?

1 GB is not that much, so you probably must tune the default configuration a bit (number of processes, cache settings of database, …). A apache process on my Linux (Debian 8) takes about 50-70 MB.

How to optimize the configuration?

if you reduce the number of processes, this can be a problem if you connect a lot of clients. If apache is too heavy, nginx might be a solution which consumes less memory but I don’t know if that is possible in your OS. You can search for guides to optimize apache and database, normally they should mention some tools how you can analyze the usage and optimize your configuration.

Sometimes, the database has a few large caches which are perhaps not used and you can reduce them. There are scripts like tuning-primer.sh and mysqltuner.pl. Your system is a closed-source solution, but with some luck it’s still close enough to most default Linuxes that all these scripts run properly.