A self-consistent mean-field theory (SCMFT) study on the phase behaviors of AB-diblock copolymer solutions is presented. All of the possible thermodynamically stable ordered structures verified by experiments are considered and the phase diagrams are generated. This work mainly focuses on the cases of selective solvents. The results show that a series of phase transitions Occurs upon dilution. In particular, large windows or lamellae + cylinder coexistence are located for the cases of highly asymmetric diblock copolymers in strongly selective solvents, which is in agreement with previous experimental observations, The transitions between the "inverted" fcc structure and the "inverted" bcc structure are also observed. The calculation indicates that the copolymer-solvent interaction plays important roles in these transitions.