Recently, color reproduction stages are developed greatly, such as liquid-crystal displays, LCD TV, LCD projectors, DLP projectors, and etc. Wide-color-gamut displays are distinguishing feature of many display manufacturers. Many researches about multi-primary color displays are proposed, but there are still some problems which are not solved. This study proposed a novel multi-primary projection display system using two projectors. One of the two projectors is modified by changing two dichroic mirrors inside. The modified projector is combined with the other to a new six-primary color display. This study applies equal-luminance boundary theorem to construct gamut volume and evaluates the merit between gamut volume and brightness. By this method, the cut-off wavelength of dichroic mirrors can be found out. In the past, to align the images of the two projectors is pre-distorted to compensate the trapezoidal distortion. This study proposes to eliminate trapezoidal distortion by using the offset of the projector. This study directly changes dichroic mirrors to maintain the brightness and contrast, and solves lower brightness and contrast resulted from adding filters before. Additionally, this study uses a reflection mirror to twist projection path and also constructs a stage to align projection images more accurately.