The phase behavior of binary blends of a long symmetric AB diblock copolymer and a short asymmetric AB diblock copolymer is studied using the self-consistent mean-field theory The investigation focuses on blends with different short diblocks by constructing phase diagrams over the whole blending compositions and a large segregation regime The influences of the chain length ratio (R) of the long and short diblock copolymers on their miscibility and on the stability of various ordered structures are explored The theoretical results reveal that the blends have a much more complex phase behavior than each constituent copolymer With the increase of the volume fraction of the short diblocks in the blends multiple transitions from a long-period lamellar phase to phases with nonzero interfacial curvatures including cylindrical and spherical phases and finally to a short-period lamellar phase or disordered phase are predicted In particular consistent with experiments, the theory predicts that the cylindrical phase is stabilized over a wide blending compositions region in the strong segregation region even though the two constituent diblock copolymers are both lamella forming When the ratio R is large enough macrophase separation occurs over a wide range of blending compositions in a relatively strong segregation regime Various coexisting phases, including those of lamellar and disorder lamellar and cylindrical cylindrical and cylindrical cylindrical and disorder spherical and disorder and cylindrical and spherical are predicted In addition, the density profiles of the typical ordered structures are presented in order to understand the self-organization of the different copolymer chains