In order to compensate the unbalanced moment introduced by POS into an aerial 3-axis ISP system, a balance weight unit is put forward. Based on the structure characteristics of ISP, an initial sample of balance weight unit is first designed. To minimize the mass and maximize the compensating moment meanwhile, the topological optimization on the connecting frame is followed via FEM that leads to the mass reduced as much as 60%. Then, the design optimization on size parameters about whole unit is carried out further until a satisfied structure gotten. The results show that the mass of the optimized structure is decreased more than 5.5% than before under the condition of compensating moment keeping unchanged.