Abstract: The fiber laser-arc hybrid welding was conducted on low alloy high strength steel(D406A). The effect of fit-up gap on the properties of D406A low alloy high strength steel welding was studied under the optimal process conditions. The results showed that the optimal fit-up gap was ranged from 0.6 to 1.0 mm. In addition, under optimum conditions, the shape of the weld, weld joint microstructure, hardness and mechanical properties was studied. The results indicated that weld metal can be divided into the arc zones, laser-arc hybrid zones and the laser zones. The microstructure of laser weld zones was tinier than the arc zones while the highest hardness of the three regions was in the heat affected zone(HAZ). After heat treatment, the mechanical properties of welded joint and base metal were nearly the same.