The effects of laser power, welding current and optical wire distance on the relative melting rate and the relationship between the joint penetration depth and the melting rate under different conditions were studied by using laser-MIG composite welding method. The experimental results show that the optical distance has the greatest influence on the relative melting rate. When no parameter is considered, the relationship between the joint penetration depth and the relative melting rate is disordered. Under the same laser power condition, greater welding penetration means greater relative melting rate can be obtained.