Is it ok as a junior PhD student to review ranked conference papers? I am a Ph.D. student in computer science and still in my first year. My professor just assigned a review task of a A-ranked conference to me in order to review research papers submitted by original authors (almost 8 papers). 

So far, he wants me to take over the coordination with my fellow Ph.D. colleagues in my department to finish this task asap. So I have to distribute the papers with the right colleague according to his/her research interest and experience. 

But, since I am still in the first year (even I am doing well 'as my colleagues say' and published two research paper in peer-reviewed conferences), I want to ask if is it common and normal thing in academia to let junior researchers review high-quality A-ranked conferences research papers, and whether the conference editorial chairs know about this conduct from assigned committee members as external reviewers.
Yes, this is pretty normal. Many conferences even have a "sub-reviewer" field in the submission form where the professor can note which of their students actually did the review.

The professor should conduct some quality assurance however. In my previous group we had a meeting to talk over our reviews, in my current group the professor takes our comments and checks them over before submitting them himself.