The mechanism of polystyrene submicrospheres assembling in aqueous suspension was studied systemically. During the evaporation of solvent, the PS spheres were transported to the water-air inter-surface, where these spheres self-assemble 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 changes 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 increases gradually. The experiment showed that only after the contact angle reached a critical value θ′critical, could the PS sphere-assembly occur. The critical value θ′critical obtained from the Wenzel equation was greater than the measured value θcritical＝22° because the surface of the PS colloidal crystal was “regularly” coarse. The change of the PS spheres at water-air interface from the hydrophilic to hydrophobic is essential for the PS spheres to assemble into colloidal crystals.