The utility model provides a but economizer that variable cross section was adjusted, it relates to an economizer, concretely relates to but economizer that variable cross section was adjusted. The utility model discloses a solve present economizer and have that actual heating load is on the low side in a short time, do not reach boiler optimum load index, this all can lead to the fact the boilerrear portion to receive hot side economizer deposition, air preheater deposition dewfall leads to the fact the stifled ash of sack cleaner dewfall even, the problem of direct influence boiler safety operation. The utility model discloses an economizer main part, a plurality of baffle and a plurality of regulation pipe valve, a plurality of baffle side by side and in parallel set up in the economizer main part, constitute a flue gas passageway between two adjacent baffles, and every flue gas passageway all is equipped with a regulation pipe valve towards the entrance that the flue gas got intothe direction. The utility model belongs to the boiler production field.