In order to realize the gap measurement between the mask and the substrate in the surface plasma lithography machine， a gap measurement method of the mask-substrate gap was proposed based on white light interferometry， and a signal processing system was designed based on ZYNQ. ZYNQ''s on-chip ARM was used for parameter setting， drive control， and front-end display. The programmable logic resources were used to realize wavelet processing and cross-correlation demodulation of spectral data. This system operated in a distributed parallel structure， which could greatly improve the detection speed， and realize the real-time measurement of the mask-substrate gap. Finally， in order to determine the measurement accuracy， a laser interferometer accuracy test platform were built. Test experiments showed that the repeat measurement accuracy was 4.4 nm， and the displacement measurement accuracy was 6.7 nm. It could satisfy the gap measurement system requirements of stabilization and higher precision.