Yingluck Shinawatra said she found the new constitution unacceptable because it is not linked to the people.
She made the comment when asked by reporters at former PM Banharn Silpa-archa's house on Tuesday.
"I still hope the National Council for Peace and Order and the National Reform Council can change it and that's a good thing to do.
"What the people want is empowerment and the right to make their own decisions," she said.
The former prime minister added an undemocratic constitution would halt the country's progress. "And I, who came from elections, did not agree if the charter is pushed forward."
Ms Yingluck brought flowers to Mr Banharn for his 83rd birthday, also on behalf of her brother Thaksin and brother-in-law Somchai Wongsawat, both former PMs.
She said it was too soon to tell whether her Pheu Thai party would campaign for a no-vote if a national referendum on the draft was held.
The charter draft is being deliberated by the National Reform Council, which will vote whether to endorse it on Sept 6.
If it votes yes, a national referendum will be held, probably on Jan 10 next year.
But if it turns it down, a new team will be appointed by the junta to write a new charter within 180 days.
"I urge more options for the people, not just a yes-no vote. For example, good points in past constitutions can be integrated or the draft can be tweaked further," she said.
But according to the 2014 charter, the draft can no longer be changed. The NRC can vote either a yes or no early next month.
"In any case, I would like the roadmap to be clearer, not extended. The longer it is, the more pain people will have to endure, especially low-income people, who have little tolerance [for economic difficulties]," she added.
Mr Banharn gave a torch to Ms Yingluck in return as a souvenir "to help light the way".
He told reporters he had not read the charter draft and was not going to.
"Whatever will be, will be. If it's promulgated, we'll have to comply with it. Whether it's right is another story.
"I don't want to criticise it. After all, Borwansak [Uwanno, the chairman of the charter writers] and I are friends," he said.
"Democracy is interpreted differently. He said it is [democratic] and I said it isn't. Suffice to say when it's endorsed, we'll run in the election. Whether we'll win is another story," Mr Banharn, chairman of the advisory board of the Chartthaipattana party, said.
"There's no use criticising the charter draft. It's not like they are going to change it for us.
"I can't say whether Thai politics is going forward or backward. We keep moving back and forth but there's nothing we can do about it. I'm old and I start to lose interest. And I won't run in the next poll."
Critics have said the junta's roadmap and how the charter draft turns out to be leave people with little choice — they either have to accept an undemocratic charter or stay under the military rule for at least two more years.
Supporters of reforms before election and the military government also want people to reject the charter but for a different reason — so Gen Prayut Chan-o-cha can lead the country a while longer.