The mechanism of the helicopter rotor whirl instability in the forward flight is investigated by the parametric resonance analysis. By comparing the system natural frequencies with the frequency components of the aerodynamic forces that are observed in the SH-60K simulation, it is shown that the particular frequency components might cause the parametric resonance. By applying the aerodynamic forces with the particular frequencies to the right-hand side of the equations of motion, the method of the variation of constants is used to evaluate the parametric resonance. These results show that the parametric resonance between the backward whirl mode and the scissor mode makes the SH-60K whirl mode unstable. In order to suppress the whirl instability, the reduction of the rotor rotational speed or the stiffening of the rotor support structure is considered to be effective.