Amorphous RuO
x (a-RuO
x) with disordered atomic arrangement and abundant coordinatively unsaturated Ru sites possesses high intrinsic electrocatalytic activity for oxygen evolution reaction (OER). However, the a-RuO
x is prone to fast corrosion during OER in strong acid. Here, we realized the stabilization of an ultrathin a-RuO
x layer via constructing heterointerface with crystalline
α-MnO
2 nanorods array (MnO
2@a-RuO
x). Benefiting from the strong electronic interfacial interaction, the asformed MnO
2@a-RuO
x electrocatalyst display an ultralow overpotential of 128 mV to reach 10 mA cm
-2 and stable operation for over 100 h in 0.1 M HClO
4. The assembled proton exchange membrane (PEM) water electrolyzer reach 1 A cm
-2 at applied cell voltage of 1.71 V. Extensive characterizations indicate the MnO
2 substrate work as an electron donor pool to prevent the overoxidation of Ru sites and the OER proceeds in adsorbent evolution mechanism process without involving lattice oxygen. Our work provides a promising route to construct robust amorphous phase electrocatalysts.