The National Council for Peace and Order (NCPO) chief said in a televised speech on Friday the interim charter draft had been completed and was being vetted by legal experts.
After the NCPO looks into it next week, the draft will be submitted for royal endorsement and then proclaimed in July.
A month later, the National Legislative Assembly (NLA) will be appointed to draft the constitution and by September the new government will begin work.
"The National Reform Council will take longer since we need to select knowledgeable and experienced people from all sectors and provinces. The process will likely take two months from the date the interim charter is proclaimed. In any case, the council will likely start work in early October," Gen Prayuth said.
"When the permanent constitution is in force, the rule of the country will be true constitutional monarchy. Even though the government appointed by the NCPO will take another three months to hold elections, the administration will be within the framework of the new charter, the result of comprehensive reforms," he said.