Laser welding of Q235 steel to 6061 aluminum alloy were weld by laser in overlap joint. In order to increase the strength, effect of laser power, velocity, defocus was investigated. The microstructure of the specimen under different parameters were observed using scanning electron microscopy and energy disperse spectroscopy. Tensile test was performed. Results obtained show that the best joints can be acquired at 850 W in laser power and 30 mm/s in welding speed. Increasing the focus amount can increase the penetration depth. The brittle phase always occurred at the interface in the joints of different power. The main component is Fe2Al5 phase, FeAl3 phase and FeAl phase.