By using a microscopic sdIBM-2+2q.p. approach, the levels of the ground-band, γ-band and partial two-quasi-particle bands for ^72-84Kr isotopes are calculated. The data obtained are in good agreement with the recent experimental results and successfully reproduce the nuclear shape phase transition of ^72-84Kr isotopes at zero temperature. The ground-state band is described successfully up to J^π=18⁺ and E_x=10.0MeV. Based on this model, the aligned requisite minimum energy has been deduced. The theoretical calculations indicate that no distinct change of nuclear states is caused by the abruptly broken pair of a boson and predict that the first backbending of Kr isotopes may be the result of aligning of two quasi-neutrons in orbit g_9/2, which gains the new experimental support of the measurements of g factors in the ^78-86Kr isotopes.