How to determine the diameter of cables is one of key problems in mega steel frame and pre-stressed composite brace structure. Firstly, through analysis of work mechanism of composite brace under horizontal load, the deciding factor of diameter value was found, and furthermore its rule was proposed. Finally, the determination equations were acquired through theoretic deduction and were proved with numerical example. The results indicate that the unbalance in composite brace resulted from antisymmetric load can be eliminated only by antisymmetric way. Thus the diameter value is decided only by antisymmetric horizontal load, and its rule is to keep the balance of composite brace under horizontal load. The influencing factor iterative leveling method can consider the influence from upper and lower floor, and is relatively accurate and feasible.