A field-induced superfluid state of magnons is exhibited in the two-dimensionalspin-1/2 quantum XY model in a strong magnetic fieldh (h/J≤2) along below a critical temperature Tc by using the renormalized field theory and the running coupling constant method. In such astate, there exist several distinctive phenomena: spins are aligned better in a constantmagnetic field as the temperature increases; and entropy rises with increasing strength ofthe magnetic field at constant temperature. Mechanisms which lead to the abovephenomena are proposed, and the distinctive curve of versus T in XCuCl3 can also be explained accordingly. Similarities and differences between our system andXCuCl3 are also discussed.