A dimensionless parameter of melting energy increment (ψ) was employed to quantify the synergic effect in laser-arc hybrid welding. The greater of the ψ, the stronger of the synergic effect. The ψ at different welding parameters of laser-tungsten inert gas and laser-metal inert gas hybrid welding was calculated and compared. The former had a stronger synergic effect than the latter because the change range of ψ was far wider than that of the latter, which was -59.3-83.6% and 1-23%, respectively. The differences were discussed according to the electrode mode and the migration behavior of charged particles in the plasmas.