The junta, known as the National Council for Peace and Order, has suggested the inclusion of a section that says if military force is "carried out with honest intention" in the interest of national security then military personnel should be exempt from prosecution.
"A blanket immunity clause in Thailand's new constitution would allow the military to commit abuses without fear of prosecution," said Brad Adams, Asia director at Human Rights Watch.
"Given the Thai military's long record of human rights abuses, this would effectively be giving soldiers a licence to kill."
The junta took power in a May 2014 military coup and a committee was appointed to draft a constitution after the previous one was torn up.
But the draft was rejected in September because of contentious points including a proposal for a national reform committee dominated by the military that allows security forces to intervene in a crisis.
A new committee is working on another draft.