An adjustable hydraulic horizontal jack including a frame, a hydraulic unit, a jib unit, link rods, and a handle. Here, the structures of the jib and the link rod(s) are improved according to an embodiment. With the improved structures, the jack has rational structures and can act reliably. In addition, under the parameters equivalent to those of conventional hydraulic horizontal jacks, the jack of an embodiment of the present invention can change the height of the lowest position and the load by adjusting the length of the jib and link rods. Therefore, the jack is adapted to jacking up not only a light automobile with a low chassis but also a heavy automobile with a high chassis, thus having a wider scope of application.