The attachment oscillator, which plays an important role in the nanophysics such as nano/microelectromechanical systems, molecular devices, and nanofibers, is studied in this work. With the help of the semi-inverse method, the variational principle is established, and the Hamiltonian of the system is correspondingly constructed based on the obtained variational principle. Then, according to the principle of energy conservation, the energy balance theory is implemented to seek for the amplitude–frequency relationship. As predicted, the obtained solution has a good agreement with the existing results, which shows that the presented method is simple but effective, and is expected to provide a new idea for the study of the nonlinear oscillator arising in the nanophysics.