For the characteristics of digital watermarking of vector spatial data, the DFT(discrete Fourier transform) blind watermarking model is put forward based on QIM(quantized index modulation) idea. Firstly, DFT is done to vector spatial data. The watermark data are embedded in the magnitude of DFT through quantification. Then, through inverse DFT transform, the embedded vector spatial data is obtained. Finally, the confirmation is done with text format's vector spatial data. The results show that the watermark has good availability, not visibility, as well as has good robustness to format conversion, data translation, rotation.