In the present work, we investigate the long-time asymptotics of the defocusing mKdV equation under a nonzero background. Using the ∂ generalization of the nonlinear steepest descent approach as well as the double scaling limit skill, the long-time asymptotics of the solution in the transition region |x/t + 6|t 2/3 ≤ C is obtained, which is associated to the Hastings-McLeod solution of the Painlevé II (PII) equation in the generic case, while Ablowitz-Segur solution in the non-generic case.