Mr Surachai, also known as Surachai Sae Dan, led "Red Siam," a Red Shirt faction.
The court found Mr Surachai guilty of lese majeste under Article 112 of the Criminal Code for speeches on several occasions that were considered insulting to the monarchy.
Mr Surachai pleaded guilty to three counts of lese majeste involving his speeches at rallies in Chiang Mai, Udon Thani and Bangkok during the 2008-2010 political upheavals.
The Criminal Court sentenced him five years in jail for each count, in total15 years, but the sentence was halved to seven years and six months after he pleaded guilty to the offences.
However, the court issued its verdict without any suspensions as they were serious offences with severe punishment and he had repeatedly committed the same act.
Mr Surachai's lawyer Karom Ponthaklang said his client has another case involving lese majeste charge.
The Wang Thonglang police were collecting evidence before forwarding the case to the prosecutor for consideration, Mr Karom said.
Mr Surachai may plead guilty when he stands trial at the Criminal Court and then he will request a royal pardon, according to his lawyer.