Wettability of water cluster on organic polluted fused quartz is studied by Molecular dynamics simulation. The stair-step Wenzel-Cassie transition critical line is obtained by on FQ surface with different pillar height and interval. The wettability decreases first and then increase as the size of cluster increases; However, the wettability decreases and then increase with the decrease of HI ratio. When the size of water cluster is close to the threshold the HI(refers to the pillar height/interval) ratio has little effect on the wettability.