The invention provides a low-frequency torsional vibrating device integrating a gear rack and a four-bar mechanism. The device comprises a torsional vibrating mechanism and a frequency and amplitude modulation mechanism, wherein a driving wheel is driven through a servo motor, and bars at two sides are driven through the driving wheel; two driven wheels are driven to rotate through the other end of each bar; two racks connected to the bars are driven through the corresponding bars; the two racks are correspondingly engaged with gears to drive the gears to realize torsional vibrating; the frequency and amplitude modulation mechanism is used for modulating the vibrating amplitude by adjusting the distance of the bars from the driving wheel end and the driven wheel ends in slots to the wheelcenters, and the frequency and amplitude modulation mechanism is capable of modulating the torsional vibrating frequency by controlling the rotating speed of the servo motor through a servo driver, thus realizing corresponding modulating of the torsional vibrating amplitude and the frequency.