Multiwalled carbon nanotubes (MWCNTs) are split along the longitudinal direction to form quasi graphene oxide (QGO) by modified Hummers method and quasi graphene nanosheet/RuO₂ (QGN/RuO₂) composites with 40.0 wt% RuO₂ loading are prepared by a one-step hydrothermal synthesis without any reducing agent. The characterization of morphology and structure shows that the amorphous RuO₂ nanoparticles are uniformly dispersed on all the surfaces of QGN. The electrochemical measurements show that the QGN/RuO₂ composites exhibit good capacitive properties compared to pure RuO₂ and pure QGN in acidic, alkaline and even in neutral electrolytes. The specific capacitances of QGN/RuO₂ in 1 M KOH, 1 M H₂SO₄, and 1 M Na₂SO₄ are 453.7, 415.7, 287.5 F g⁻¹. Significantly, the QGN/RuO₂ composites can achieve a high voltage window of 1.6 V and greatly increase the energy density (102.2 W h kg⁻¹) and power density (1600 W kg⁻¹) of supercapacitors in 1 M Na₂SO₄ (1 A g⁻¹). These results demonstrate that the QGN/RuO₂ composites are a promising material for high-performance supercapacitors.