In order to study the weld forming law of the 10 kW fiber laser self-fusion welding, and then realize the well formed weld under the condition of 30 kW laser power, the test taking30 mm and 40 mm thick Q235 low carbon steels as research object are carried out. On the basis of flat welding, the weld characteristics were analyzed under the conditions of different laser powers, defocusing amounts, welding speeds and side blow pressures. The welding parameters were optimized and the weld forming was controlled by self-made small side blow device.The results show that when the laser power reaches 30 k W, it is difficult to obtain a good weld formation only by changing the defocusing amount or welding speed, and the weld uniformity is poor and the welding spatter is large. Applying transverse side blow pressure of 0.3 ～0.4 MPa on the test plate can effectively improve the weld formation, reduce the welding spatter, and increase the weld penetration depth by more than 20%. Through the further optimization of the welding parameters, the weld surface forming control was realized during 30 kW laser welding with transverse side blow pressure of 0.4 MPa and welding speed of 0.6 m/min.