It is difficult to exposure the square unit phase structures with sizes of several microns on diffraction patterns by using Gaussian focus spot in a laser direct writing system. A novel changeable double or single beam optical head in the laser direct writing system has been designed and investigated. A method of using projection imaging optical configuration to obtain a square shape spot has been presented for improving the quality of exposed diffraction patterns and increasing the running efficiency by dot-by-dot exposure mode. Both the diffraction optical variable images with two beam interferential fringes on the dots and the binary phase encoding patterns can be simultaneously written on the same photoresist plate. The resolution of diffraction optical variable images can be achieved up to 2540 dpi, the size of square-shaped beams is adjustable within 5～20 μm. The diffraction optical variable images and binary phase encoding patterns with two levels have been fabricated. The experimental results are given.