These include lengthy detention without trial, restricted legal access and increased use of the death penalty.
But a Saudi official said it was directed at terrorists, not dissidents.
The Saudi government has so far declined to comment, but the senior official, who did not want to be named, confirmed the existence of the draft law and did not dispute the clauses contained in it.
Amnesty International's Middle East press officer James Lynch told the BBC the draft law - a copy of which was leaked to the human rights group - "seeks to entrench some of the most repressive practices that Amnesty has been documenting for years".
Among the measures proposed is a broadening of the definition of a terrorist crime to include any action deemed to be "harming the reputation of the state" or "endangering national unity".
AnalysisSaudi Arabia has fought and largely won a protracted campaign against al-Qaeda-inspired terrorism.
But the gist of Amnesty International's criticism of this new draft law is that parts of it are really more aimed at crushing political dissent than locking up dangerous terrorists.
In a country where political parties are banned and half the adult population (women) are not allowed to drive, Saudi Arabia's conservative rulers have been deeply rattled by the "Arab Spring" street protest movement.
Amnesty now fears that if this secret draft law comes into practice, it could offer the authorities dramatically enhanced powers to arrest anyone suspected of political dissent.
But a Saudi government advisor, who closely monitors terrorism cases, told me it is all down to interpretation.
He strongly disagrees with Amnesty's view. He insists this draft law is needed to fight al-Qaeda militants trying to get into the country from Yemen.
"We face", he said, "a serious ongoing threat".
Questioning the integrity of Saudi Arabia's rulers would become an offence punishable by a minimum of 10 years in prison.
Amnesty said a number of provisions in the document contradicted the kingdom's legal obligations, including the UN Convention against Torture.
Philip Luther from the charity said the law would be a serious threat to freedom of expression in Saudi Arabia in the name of preventing terrorism.
"If passed it would pave the way for even the smallest acts of peaceful dissent to be branded terrorism and risk massive human rights violations," he said.
The BBC's Frank Gardner says Saudi Arabia has fought and largely won a protracted battle against al-Qaeda-inspired terrorism in recent years.
But the gist of Amnesty's criticism is that parts of the new draft law are more aimed at crushing political dissent than locking up dangerous terrorists.