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A capacity variable watermarking algorithm for 2D engineering graphic based on complex number system is proposed. The coordinates of vertices in the engineering graphic are extracted to construct complex signals, which are transformed by Fourier transformation. The watermark sequence will be selected m bits each time to have a transformation under complex number base, then the real part and imaginary part of Gaussian integers are required, which are used to modulate the amplitude of Fourier coefficient. By modifying the size of m, the capacity of the watermark is variable. SNR is introduced as a parameter to quantitative the changes induced by watermarking scheme. At the same time, the watermark image is encrypted to improve its security. Experimental results show that the proposed algorithm is robust to the attacks such as rotation, translation, scaling, addition and deletion.