The relationship between the surface wettability and the interaction strength of liquid-wall particles is first investigated using a lattice Boltzmann method, and then the liquid flow over hydrophobic surfaces is simulated in a microchannel. Effect of surface wettability on the slip flow and drag reduction characteristics of hydrophobic surfaces is obtained. Existence of the apparent slip on hydrophobic surfaces is confirmed and its mechanism is revealed. Simulation results show that the hydrophobicity induces a low density layer near the wall of hydrophobic surfaces and the apparent slip is observed on the low density layer. It is shown that the apparent slip is a direct cause of hydrophobic surfaces' drag reduction effect. Thus the drag reduction effect increases with increasing slip length. For a specific fluid system, the slip length is an inherent property of the hydrophobic surfaces and is a single function of the surface wettability. The slip length does not change with the external flow property.