Sahlqvist axiom schema in multimodal logics is a generalization of Sahlqvist formulas in monomodal logics. In multimodal logics, the Sahlqvist axiom schema corresponds to first-order formulas on frames. The properties of the frames can be characterized using relational equations. Any normal multimodal logics containing Sahlqvist axiom schema are complete with respect to the set of first-order frames that satisfy the corresponding properties of the Sahlqvist axiom schema. The Sahlqvist axiom schema has more expressive power in multimodal logics.