Film cooling technique is widely used to protect the components from being destroyed by hot mainstream in a modern gas turbine. Combining round-holes is a promising way of improving film cooling effectiveness. A DoE (design of experiment) simulation of 396 cases focusing on the arrangement of the combined-hole with double holes for improving film cooling performance is carried out in this work, and the influence of an aerodynamic parameter, blowing ratio is considered as well. The dimensionless lateral distance (PoD) and compound angle (CA) of the double holes have relative influence on the film cooling performance of the combined-hole unit. At the low blowing ratio, increasing symmetrical compound angle (SCA) has positive influence on the area-average effectiveness (EFF) of the combined-hole. But at the intermediate and large blowing ratio, the influence of SCA on the area-average EFF depends on the range of PoD. At the small PoD, the area-average EFF ascends basically along SCA axis. However, the area-average EFF first ascends and subsequently descends along SCA axis at the large PoD. Asymmetrical compound angle (ACA) is also considered to fit the antikidney vortexes produced in the combined-hole film cooling compared to their ideal schematic. However, the film cooling effect of the cases with ACA is not as good as expected. The area-average EFF of ACA cases locates in the level between that of the adjacent SCA cases. The optimal arrangement of combined-hole unit for improving film cooling effectiveness is relative to the local flow field. The optimal arrangement of PoD and CA for improving the combined-hole film cooling performance is different at different blowing ratios.