Mark Simons was among 55 objectors banned from the Hampshire site by the High Court last July. He was given a 28-day suspended jail sentence last September for breaching the injunction.
Yesterday, Mr Justice Turner was told that Simons broke the order again in November and he saw photographs and a video recording of Simons beating a drum and watching a play performed by other protesters on the site.
Enforcing the 28-day jail term and adding three months for the additional breach, the judge said Simons returned to the site in deliberate defiance of the injunction.
Simons, who is from Somerset and has been living in squats, told the judge it seemed 'absurd' to jail him for watching a play.
The jail order was sought by the Ministry of Transport, which has fought a long legal battle with protesters. It claims delays to the project and damage to equipment have cost more than pounds 1m.