We introduce a Casimir force in a conventional optomechanical system to study the high-order sideband generation. In this system, a nanosphere is placed near the moveable mirror of the conventional optomechanical system. The moveable mirror is coupled to the cavity field and the nanosphere by the optomechanical interaction and the Casimir interaction, respectively. We find that the amplitude and cutoff order of the high-order sideband can be enhanced by decreasing the sphere–mirror separation (increasing the Casimir force) and increasing the optomechanical coupling strength. Our proposal provides an alternative method for generating the high-order sidebands and for measuring the Casimir force.