The mechanism of polystyrene submicrosphere assembly in aqueous suspension was studied systemically. During the evaporation of solvent, the PS spheres were transported to the water-air surface, where these spheres self-assembled into colloidal crystal through the capillary force between the particles at water-air interface. The wettability of the PS spheres at the water-air interface could change from complete wetting to partial wetting and finally to non-wetting with the evaporation of water, so that the contact angle between the PS spheres and water increased gradually. The researches showed that only after the contact angle reached a critical value θ′critical, could the PS sphere-assembly occur. The critical value θ′critical which was obtained from the Wenzel equation,was greate than the measured value θcritical＝22°, because the surface of PS colloidal crystal was “regularly” coarse. The change of the PS spheres at water-air interface from hydrophilic to hydrophobic is essential for the PS spheres assembling into colloidal crystals.