Orthodontic dental casts lightweight research was very important to reduce the cost of invisible orthodontics. Actual dental casts deformation was analyzed by finite element method, which were successively filled by hollow, honeycomb, and cube structures. Film lamination was equivalent to the outer surface normal load of orthodontic dental casts, whose overall deformation was measured to evaluate lightweight design. At last the new dental cast was used to verify the research results. The results indicate that honeycomb structure is best for orthodontic dental casts lightweight design, which is followed by cubic and hollow structure successively.