Some sources online listed the death toll of the An Shi rebellion as over 30 millions.
http://users.erols.com/mwhite28/warstat0.htm#AnLushan

Most sources I read estimated the Tang population as some where around 50 millions during mid-700s.

That would mean the seven years rebellion killed whopping 60% of the population.

Is it even possible in a pre-industrial society to have such a massive casualty?

Generic, this is not directed towards you but people tend to have some pretty distorted views regarding China's demographic history. Who's to say that such a drastic reduction in population was the product of mass killing and death and not of inaccurate census taking? When population declines, is it solely due to mass death from killings itself or other factors? Low fertility rates caused from economic decline, disease, internal migration, external migration, intentional census registration for tax omission purposes, etc. Granted that there was undoubtedly massive deaths due to killings from the rebellions, this was hardly the only factor as to why Chinese population dropped.

According to census figures right after the An Lu Shan rebellion Tang China's population fell from 52 million to 18 million. What you have to remember was the Tang China survived the rebellion at the expense of being highly decentralised. A lot of provinces especially in the North East became virtually autonomous. It was estimated that the Tang Court lost 25% of the Empires manpower to these autonomous provinces. Even taking into account inaccurate census taking in the autonomous provinces, its fair to say a large proportion of Chinas population died during the rebellion.

I find it unlikely that so many people were killed in just 7 years.

If more than 50% of the population were really killed, there should have been a bigger consequence than was really the case. During Europe's Black Death only 25% of the population were killed, yet it had a very fundamental and profound impact upon European history.