SH waveforms recorded by a dense regional seismic network are used to constrain the shear wave velocity structure of the upper-mantle transition zone beneath northeast China. By modelling triplication waveforms we show that a 130-km-thick layer of increased seismic velocities is overlying a broad slightly depressed 660 km discontinuity. This anomaly can be associated with the westward extension of the stagnant Pacific slab that is deflected and still cold. The transition zone, as a whole, is featured by low shear wave velocity and high Vp/Vs ratio, which infer H2O content of ∼0.2–0.3 wt. per cent [(3.0–4.5)×104 ppm H/Si] atop the 660 km discontinuity. We interpret that the fast eastward retreat of the Japan Trench facilitated the slab flattening when the subducting oceanic lithosphere hit the bottom of the transition zone, and the leading edge of the slab is currently trapped in a water-bearing mantle beneath northeast China.