Precision optical systems that utilize laser beams as working media usually suffer from thermal aberrations caused by absorbed energy. Based on a specially designed three-lens system, the causes and contributions of mechanical structures to the system's thermal aberrations are studied. The contribution of three thermal effects, surface deformation, change of refractive index, and stress birefringence on the system's thermal aberrations, is analyzed respectively through an integrated optomechanical simulation method. The impact of the structure's thermal dissipating capability and structure configuration on the system's thermal aberrations is analyzed, too. Experiments have been carried out to validate the correctness and accuracy of the simulation method. Both the simulated and tested results can provide a reference for structure design and thermal aberration analysis of the similar optical systems.