The invention discloses an online-controllable-atmosphere feedback device for preparing glycolaldehyde by using synthesis gas and an application thereof, and belongs to the technical field of glycolaldehyde production. The device comprises a gas control system A, a pressurization system B, a reaction kettle system C, an analysis system D and a central processing system E. The device solves the technical problems of a conventional high pressure reaction device for preparing glycolaldehyde by using synthesis gas: 1, the raw material gas proportion is relatively fixed, the change period is long, and the raw material gas proportion cannot be adjusted targetedly according to a rhodium precursor component; 2, the reaction evaluation data analysis period is long, and adjustment and change of the raw material gas proportion of the reaction process cannot be timely fed back and guided; and 3, gas ways are frequently dismantled, potential safety hazards are increased, and the work is tedious.