In order to compare the welding characteristics of fiber laser and CO2 laser, a 2 kW fiber laser and a 2.4 kW CO2 laser are used to weld 590 MPa high strength steel plates. By changing the defocusing distance and the welding speed, the penetration depth is investigated. The results show that with the same welding speed and the same output power, the fiber laser welding gets more penetration depth compared with CO2 laser welding. With a negative defocusing distance, the penetration depth becomes greater when the defocusing distance is within the range of focal depth; the penetration depth increases with the decrease of welding speed when the power is constant, and the penetration depth using the CO2 laser welding decreases faster with the increase of welding speed than fiber laser welding.