I want to travel to Guangzhou for 1 week. is it compulsory to apply for VISA. I am UAE citizen.

Usually, VOA is not available there. If you transit in Guangzhou for less than 3 days, you can enjoy the 72 hour free transit there. In this case, no entry permit is needed.
But as you said, you will go there for a one-week visit, so you need to apply for a tourist L visa ahead of your trip.

What is documents required to enter Guangzhou? I heard that I have to get invitation from someone there? Is it compulsory? I only visit for tourism why the invitation required?

You need to provide the application form, a passport-style photo, your passport and its photocopy, round trip air tickets and accommodation booking record. The invitation letter is not necessary if you provide the air tickets and hotel reservation.