Abstract: Inter-reflection is a common phenomenon in projection display in concave environments, which may cause non-uniform radiance and color deviation. Since the materials of most natural projection surface are non-Lambertian, the inter-reflection problem is more complex than Lambertian surfaces. In this paper, an inter-reflection compensation method for projection display in concave non-Lambertian environments is proposed, base on the analysis of inter-reflections among non-Lambertian surfaces. Firstly, an inter-reflection model is built to describe physical light transport in a concave non-Lambertian environment. Secondly, a non-negative least squares method is applied to optimize the input value of pixels to make the final result closer to the desired displayed images, and then the impact of inter-reflection can be reduced. Thirdly, a reflection parameter estimation method is proposed to estimate the value of parameters in the inter-reflection model, then applied to a folding screen and a dome. Experimental results demonstrate that the proposed method can effectively reduce the impact of inter-reflections in concave environments.