Searching for highly-efficient electrocatalysts for water splitting has been greatly endowed due to the huge demand for green energy sources. Two-dimensional (2D) materials are widely explored for the purpose because of their unique physical and chemical properties, abundant active sites, and easy fabrication. Here, we present a new family of 2D M₄B₆X₆ (2D Boridenes) and investigate their physical and chemical properties for their potential applications into electrocatalysis based on first-principles calculations. We demonstrate that 2D M₄B₆X₆ (M=Cr, Mo, and W; X=O and F) are dynamically, thermodynamically, and mechanically stable, and show intriguing electronic and catalytic properties. Importantly, we find that M₄B₆O₆ are intrinsically active for oxygen evolution reaction (OER). Our results demonstrate that: (1) the adsorbate-escape mechanism dominates the OER process with a low overpotential of 0.652 V on Cr₄B₆O₆; (2) the partial surface-oxidization can improve the catalytic performance of M₄B₆F₆ dramatically; and (3) the surface reconstruction greatly affects the OER performance of M₄B₆X₆. Our findings illustrate that the surface reconstruction is critical to the OER activity, which may provide a new strategy on the design of 2D materials for electrocatalysis and offer theoretical insight into the catalytic mechanism.