To study the impacts of environment luminance, chromaticity of background and interferences (including specular reflection, diffuse reflection and glare) on the readability of Head-Up Display (HUD), a readability evaluation model was established based on Perceptible Just Noticeable Difference (PJND) theory by taking the characteristics of HUD and its environment into consideration.Simulations and experiments result shows that:1) The value of PJND can be used for evaluating the readability of HUD;2) Low-luminance environment has a higher luminance contrast requirement than high-luminance environment;To satisfy the readability, the HUD needs to lower the luminance contrast when the ambient luminance increases;3) The bigger the difference between background chromaticity and object chromaticity, the lower the requirement to luminance contrast;and 4)When the environment luminance is constant and the interferences increase, a higher luminance contrast can be used to satisfy the readability;when the interference luminance is beyond the maximum, physical occlusion is a way to satisfy the readability.