An innovative way was proposed to detect citrus canopy LAI fast and easily based on wide-angle photographing technology. The citrus canopy images obtained by wide-angle lens were first sent into computer. And then the images were grayed, enhanced, filtered and binarized. The system calculated the citrus canopy gap fraction and derived LAI. The citrus canopy LAI detected by scanning method was taken as true value in the whole experiment. The Canadian WinSCANOPY was used in comparison with the system in detecting effect. The result of data collection experiment showed that the variation range of relative error was -30.39%~43.53% and the average of absolute relative error was 18.90% by the Canadian WinSCANOPY. The variation range of relative error was -27.26%~37.06% and the average of absolute relative error was 19.91% by the system based on same test condition. This system meets test requirements of citrus canopy LAI detecting.