Inthis paper, we prove that if M^2 is a complete maximalspacelike surface of an anti-de Sitter space {\bfH}^{4}_{2}(c) with constant scalar curvature, then S=0,S={-10c\over 11}, S={-4c\over 3} or S=-2c,where S is the squared norm of the second fundamental form ofM^{2}. Also
(1) S=0 if and only if M^2 is the totallygeodesic surface {\bf H}^2(c);
(2) S={-4c\over3} if and only if M^2 is the hyperbolic Veronesesurface;
(3) S=-2c if and only if M^2 is the hyperboliccylinder of the totally geodesic
surface {\bfH}^{3}_{1}(c) of {\bfH}^{4}_{2}(c).
1991 Mathematics Subject Classifaction 53C40, 53C42.