An Islamist militant group called the Turkestan Islamic Party claimed responsibility for the attack at a train station in Urumqi late last month that killed one and injured 79 people, the SITE Monitoring service said.
The authorities had said the attack in Xinjiang, home to the Muslim Uygur ethnic group, was carried out by two religious extremists who were also killed in the blast.
Xinjiang has been beset by violence for years and a recent series of attacks, some of which Beijing has called terrorism, have unnerved the country.
SITE, which tracks Islamist militant statements, said the Turkestan Islamic Party had released a 10-minute video in the Uygur language showing the construction of a briefcase bomb it said was used in the station attack.
"A fighter is shown placing the explosive material and shrapnel of bolts inside a box, then inserting the detonation device in a briefcase with the explosive and leaving the trigger exposed in an outside pocket," SITE said.
Beijing says it faces a real threat from militant Islamists in Xinjiang who want an independent state called East Turkestan.
The authorities say many have links with foreign groups, although rights groups and some foreign experts say there is little evidence to support this. The Turkestan Islamic Party, which Beijing equates with the East Turkestan Islamic Movement, keeps a low profile.
Unlike the Taliban, it rarely posts videos promoting its activities or ideology.
Foreign Ministry spokeswoman Hua Chunying said she had not seen the video, but she believed it might further bolster China's position on separatist groups.
"Some violent extremist terrorist groups have now emerged. They are colluding with foreign groups and are attempting violent activities in Xinjiang and other areas in China intended to destroy China's national policy and social stability," Hua told a regular press briefing. "We hope that everyone can recognise the goal of these violent groups and support the Chinese government's will to crack down on all violent terrorist activities," she said.