The cap should be placed on sales to provider's existing customers to ensure people are protected from being sold inappropriate products, once the new rules kick in, it said.
It made the call after finding the drawdown market's costs vary significantly.
The suggestion is part of its move to launch a campaign calling for 'Better pensions'.
It said there was still a lack of product innovation and trust in the drawdown market, with big variations in charges between companies and even within the same provider.
The Which? report has been launched in the wake of the Financial Conduct Authority warning people about the risks in cashing in their final salary pensions under the new freedoms.
The Which? report stated?: "In a snapshot investigation of income drawdown products currently on the market, we looked at how the reforms might impact a consumer who uses their existing provider's products, and found the costs vary significantly.

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"Our research uncovered several high charging drawdown products, including one that charges 2.76%. We think this is too high for the mass market and want to see a cap introduced on products sold to customers by their existing provider.
"Based on a scenario of someone with the typical pension pot of £36,000, drawing down £2,000 a year, we calculate that a cap of 0.5% would leave someone in our scenario around £10,300 better off than with charges at 2.75%.
"A 0.75% cap would mean that they have a total of around £8,800 more over their retirement and a 1% cap would give them around £7,500 more.
"We also found one provider charging 0.5% more than another for investing in the exact same fund, and one provider's charges ranging from 0.44% to 1.24% for very similar funds, which can make a significant difference over the course of someone's retirement."
Which? said it wants a Government-backed drawdown scheme to be established to "give everyone access to a good value, low cost product no matter who their pension provider".
Which? executive director, Richard Lloyd, said: "It's right that the Government is giving people the freedom to decide how and when they access their hard-earned pension savings, but deciding how to use these savings in retirement is one of the most complex financial decisions many will have to make and one they cannot afford to get wrong.
"That's why we want the Government to take action to secure better pensions so people have just as much protection when they take money out of their pension as when they put money in."