The invention discloses a geogrid unreeling device and an operation step thereof. The geogrid unreeling device comprises a device base, an unreeling drive mechanism, an ureeling frame, a lifting chuck, an upper device cover, a cutoff tool head, a grid outlet drive mechanism, a tensioning mechanism, a caterpillar drive mechanism, a horizontal movement device and a device casing. The length of geogrids can be controlled through arrangement of a cutting mechanism, the sampled geogrids meet the project requirements, besides, unnecessary waste is avoided, fewer geogrids are used, and the cost is reduced; by means of the caterpillar drive mechanism, the geogrid unreeling device can adapt to different environments; by adopting a motor for driving, labor is saved and the unreeling efficiency is higher; by additionally arranging a tensioning device, grids inside are prevented from loosening and twining around the geogrid unreeling device to result in damage to the device; the geogrid unreelingdevice has higher operation efficiency next time by means of the cutoff tool head with a lock seam; and the geogrid unreeling device not only can avoid unnecessary waste, reduce the number of the geogrids and reduce the cost, but also has high unreeling efficiency.