Abstract: It is difficult to coat photoresist uniformly on large plates for diffraction optics component fabrication. The relationship between photoresist thickness and scan velocity is experimentally explored by a small meniscus coating applicator. Aside from the thickness, the distribution uniformity of the thickness is also important and is experimentally found it is mainly dependent on system stability. Firstly, the relationship between the thickness uniformity and the scan velocity is explored and optimized. Secondly, interspace variations during the scan process with different coating parameters are measured in situ using a non-contact laser triangulation sensor, it is limited to 15 m in the scan process. Several variation reduction measures are adopted. After parameter optimization and system improvement, the photoresist is uniformly coated with a peak-to-valley (PV) value of 3% and a root-mean-square deviation of 0.5% to fabricate diffractive optic plates.