When biological cells or tissue thick specimens are imaged by bio-optical microscopy, image resolution decreased significantly due to the impact of diffraction and defocus. Digital confocal microscopy can improve the image resolution through a three-dimensional image restoration method. The criterions of resolution degradation ratio and resolution improvement ratio are proposed, and a simulation-thick specimen is designed with different lateral and axial spacing of optical points and lines. The full width at half-maximam is used for measurement, evaluation and analysis in the image resolution of thin specimen diffraction imaging, thick specimen imaging containing out-of-focus ingredients, and image by restored digital confocal microscopy. The results show that the digital confocal microscopy has high resolution improve ratio, and can restore lateral and axial resolution of thick specimen effectively.