The court found Sonam guilty of inciting people to riot and leading attacks against police cars and stores.
The other five defendants were accused of protecting him after the incident, providing him with material help when he was on the run.
Sonam, 23, was born in semi-nomadic family in Rachap Township, in the Kardze Tibetan Autonomous Prefecture.
He was arrested in October 2009, 17 months after the Lhasa protests, and was considered a protest leader.
So far, 450 people, all ethnic Tibetan, have been convicted by Chinese courts for the violence that broke out during anti-Chinese protests a few months before the Beijing Olympics.
For Chinese authorities, the people involved are “common criminals” or “subversive elements” who used the unrest, caused by Han Chinese domination and anti-Tibetan discrimination, to inflict damage to people and property.
Chinese propaganda and Chinese authorities have stressed the material damage caused by the protests in April 2008 in order to highlight their impact on individuals for the benefit of the international community.
The Dalai Lama and Tibetan leaders have conversely stressed the non-violent element in the events.