The ringing of a floating barge’s roll motion under focused wave groups is studied using a fully nonlinear method in a numerical wave tank based on the potential flow theory. The higher order boundary element method (HOBEM) is used to solve the boundary value problem. The mixed Eulerian–Lagrangian (MEL) technique and the fourth-order Runge–Kutta time stepping scheme is used to simulate the interaction between the floating barge and focused wave groups. As the peak spectra frequency is set as one-third of the natural frequency of the barge’s roll motion, the influence of the peak wave amplitude of the incident focused wave groups on the roll response of the barge and wave loads is studied. The third-order components are found to be significant in both roll motion and moment as the incident peak wave amplitude increases. Relationship between the roll response of the barge and the peak spectrum frequency of the incident focused wave groups is also studied. The peak of the third-order roll response of the barge is found to decrease greatly as the triple peak spectral frequency deviates from the roll natural frequency.