The invention relates to the field of microalgae culture and specifically relates to a trap-type carbon supplement device and carbon supplement method for cultivating microalgae in an open pond. The trap-type carbon supplement device for cultivating microalgae in an open pond, comprises a trap-type container, a partition plate and a gas distributor, wherein the gas distributor is positioned at the culture solution inlet of the trap-type carbon supplement device; the thickness of the trap-type carbon supplement device on the side of the culture solution inlet is 0.5-2 times of the depth of the culture solution in the open pond; the gap between the lower end of the partition plate and the bottom of the trap-type container is 0.5-2 times of the thickness of the trap-type carbon supplement device on the side of the culture solution inlet; the upper end of the partition plate is higher than the wall of the trap-type container; and the width of the partition plate is matched with the trap-type container. The carbon supplement device of the invention can make the gas-liquid contact time longer and reduce the depth of the trap-type container, therefore it can reduce the flow resistance of the liquid in the carbon supplement device and save energy consumption.