Mr Li is on an official visit to France where a UN-led climate summit will be held later this year.
After meeting officials in Brussels, he said the details would be announced by the end of the month, which is Tuesday.
Last November, China - the world's top greenhouse gas emitter - committed for the first time to limit its output.
Chinese President Xi Jinping said the country would target 2030 for its emissions to peak.
At a press conference in Brussels on Monday after meetings with EU leaders Donald Tusk and Jean-Claude Juncker, Mr Li said China would submit its Intended Nationally Determined Contribution (INDC) report to the UN secretariat on climate change "by the end of this month".
All countries are supposed to submit their INDC, as the pledges are called by the UN Framework Convention on Climate Change, before the Paris summit in November.
The summit is aimed at reaching a pact to limit global warming to 2C (3.6F) over pre-Industrial Revolution levels, to take effect from 2020.
The European Union is eager to avoid what happened at the Copenhagen UN summit in 2009, which failed to reach a deal in part because of China's reluctance to make a binding commitment.
Also on Monday, Mr Li called on Greece and its creditors to reach a last-minute deal and avoid an exit from the eurozone.
"Whether Greece stays within the euro is not only a question that concerns Europe, but also concerns China and Europe," Mr Li said.
"In addition, this is also something that concerns world financial stability and economic recovery."
China is the European Union's second-biggest trading partner.
China is set to reveal how much it intends to cut its carbon emissions by beyond 2020, Chinese Premier Li Keqiang has said.