The phase behavior of double-hydrophilic AB diblock copolymers in concentrated aqueous solutions is investigated using a simulated annealing technique. Phase diagrams of the system are constructed as a function of the volume fraction and concentration of the copolymer (Phi) as well as the hydrophilicity difference between the two blocks. Rich phase transition sequences, especially reentrant phase transitions, such as lamellae -> gyroid -> hexagonally packed cylinders -> gyroid -> lamellae -> disorder, are observed for a given copolymer with decreasing Phi. By analyzing the variations of the average contact numbers between the A or B monomers and solvents, and of the effective volume fractions, the mechanisms of the reentrant, the order-order, and the order disorder transitions are elucidated. The difference in hydrophilicity or in volume fraction can be used to tune the degree of swelling of the two blocks, resulting in a nonmonotonic variation of the effective volume fraction of the A (or B)-rich domain with the decrease of EI), thus inducing the reentrant transitions. Our results are compared with those from available experiments, theory, and simulation and also with the simulation result of an amphiphilic diblock copolymer.