Technical problems like difficulty of fusion and brittle intermetallic compounds between Fe and Al are urgently to be solved in order to get high quality steel-to-aluminun weld seam with high efficiency. Low-carbon steel and aluminun alloy sheet (thicknesses are 5 mm and 6 mm, respectively) are welded in butt way by a fiber laser. Welding conducted with Si powder as the filler placed in the end before welding is also researched. Good welding beads have been obtained through adjusting the process parameters and pre-filling Si powder. The welding bead is detected and observed by metallurgical microscope, scanning electron microscope, X-ray diffraction and some other ways. The results indicate that dissimilar steel and aluminun can be connected by laser welding under heat conduction welding mode under the conditions that laser power is 1.8~2.0 kW, welding speed is 8~10 mm/s, defocusing is -2 mm, protective gas is Ar, gas flow rate is 15 L/min and offset to crevice between steel and aluminun is 0.2 mm. The grains in weld zone are fine. Micro-hardness in weld zone is higher than those in heat affected zone and base material. The boundary between steel and aluminum is clear. Two base materials are combined by the molten metal, embedded in each other like gears. The quantities of the melting metals Fe and Al in the pool are controlled. The fluidity of the pool is improved by the Si powder filler and it is good for melting metal to spread out in the end of the sheets. Compounds of Al9Si and Fe0.9Si0.1 with better stability are found and the generation of intermetallic compounds of Fe-Al is inhibited.