The utility model discloses a height-adjustable installing support of a derrick, which comprises an upper support body, a lower support body and a lifting device. The upper support body comprises two girder structures which are formed by erected rods, sleepers and beams. Two support rods are symmetrically arranged along the lower surface of the beams, and the erected rods are provided with pin holes at identical intervals. The lower support body comprises a left stand column and a right stand column which are respectively provided with a short beam. The lower edge of each short beam is provided with an ear plate, and a dead line ear plate is arranged on a forklift beam. A lifting system comprises a puffer, the lower ends of the support rods are provided with a lifting pulley, the lower surfaces of the beams are provided with a fixed pulley, and a rope led out of the puffer penetrates through the fixed pulley, a movable pulley connected with a tripod and the movable pulley and is fixed on the dead line ear plate. A fixed-length rope penetrates through one side of the ear plate, the lifting pulley and the tripod and returns into the other side of the lifting pulley and the ear plate. The height-adjustable installing support of the derrick achieves the aim of replacing a high support and a low support with one support, is easy to move and has low use cost and high work efficiency.