A decoder, a projecting system, and a method for processing an image thereof are provided. The decoder is adapted to the projecting system having a projecting module and a shutter glass. The decoder includes a decoding circuit and an indicating unit. The projecting module outputs a frame turning control signal to the shutter glass. The decoding circuit receives and decodes a three-dimensional (3D) video signal to produce a first image data and a second image data to the projecting module. The indicating unit is coupled to the decoding circuit and correspondingly produces an indicating signal when the decoding circuit outputs one of the first image data and the second image data.