A method for fabricating spiral phase plate (SPP) with continuous surface is proposed. An interval is used to quantize the three dimension surface of SPP into two dimension bars to form a binary mask. The exposure dose can be precisely modulated through this mask in the exposure process. The select criterion of the quantization interval is discussed in detail. As the results, four kinds of high-quality SPPs with topological charges 1,3,10 and 20 are fabricated which can be used under the wavelength of 532 nm. The corresponding optical effects are also measured.