We have performed ab initio molecular-dynamics simulation of liquid GaSb (l-GaSb) up to 20.0 GPa. The calculated structure factors are consistent with the recent experimental results, and the partial structure parameters show that the structure of l-GaSb under pressure contracts nonuniformly. In the whole calculated pressure region, the contraction of l-GaSb can be divided into three substages: 1.8-5.4, 5.4-10.0, and 10.0-20.0 GPa. It is further confirmed by analyzing the bond-angle distributions of Ga-Ga-Ga and Sb-Sb-Sb that the rearrangement of Sb atoms under pressure plays a crucial role in the structure change of l-GaSb.