To enhance the vibration damping performance of traditional magnetorheological dampers in the low-frequency domain, a novel magnetorheological inertial device is proposed. It consists of a variable damping unit and an inertance unit, incorporating a magnetorheological (MR) valve, hydraulic motor, and flywheel. The relative movement between the piston rod and cylinder facilitates the reciprocating flow of the MRF within the variable damping valve. The output damping is adjusted by controlling the current magnitude on the coil. Simultaneously, the oil drives the hydraulic motor output shaft and the rotation of the flywheel, realizing the inertance characteristics of the device. To maximize the magnetic field utilization within the built-in channel of the MR valve, an optimized magnetic circuit design approach is proposed. Finite element analysis is utilized to simulate and analyze the internal magnetic field intensity of the MR valve. Combining magnetic field simulation with theoretical calculations, a prototype is designed and fabricated. A performance testing platform for the device is established and tested. The results indicate that the designed device exhibits inertance characteristics and has excellent controllability of damping.