•Fluid density functional theory can accurately simulate the structure of grafting polyelectrolyte solutions at interfaces.•The influence of chain length and density of the grafted polyelectrolyte on interface structure and capacitance was revealed.•The grafted polymer brush disrupts the alternating layer of anions and cations at the conventional interface. The structure of the electrode/electrolyte interface directly determines electrochemical performance. One effective method for controlling this interface structure is by grafting polyelectrolyte onto the electrode surface, which can significantly alter the double layer structure and enhance electrochemical performance. This study focuses on the graft modification of polyelectrolytes at the electrochemical interface and investigates the impact of the length and density of the grafted polyelectrolyte chain on the structure and capacitance of the electrode/ionic liquid interface using fluid density functional theory (FDFT). The findings indicate that the grafted polymerbrush disrupts the alternating layer of anions and cations at the conventional interface and exerts an additional adsorption effect on counter ions, thereby reducing the density of freely movable co-ions. Consequently, this process increases the effective charge and capacitance of the interface. The results of this investigation contribute to a deeper comprehension of polyelectrolyte solutions at solid–liquid interfaces and guide the regulation of electrochemical interface properties.