The proposals, which will be fleshed out by the Chancellor in this week's Budget, would provide a 50% bonus to those who manage to save up to £50 a month.
As many as 3.5m people in low paid work and receiving working tax credit or universal credit would qualify, with the maximum level of help set at £1,200.
However, they will have to save for two years before getting any bonus.
Stepchange says such a wait may see families overtaken by events as they dip into funds for emergencies.
The charity warns that the thought of losing the bonus could act as a disincentive to save in the first place.