Grating moving light modulator (GMLM) is a novel MEMS-based movable diffraction grating with electrostatic actuation, which light modulation is completed by changing micro distance between movable grating and underlying fixed reflector. The micro distance is a key factor to GMLM performance and it is difficult and expensive to measure with a common approach. A novel method using spectrum analysis method based on wavelength scanning is put forward and analyzed in details. Experiments with U-4100 spectrophotometer are carried on, which indicates that such method is available and high repetition. Such wavelength scanning method can measure micro distance in diffraction or interference-based micro module. More important, measurement accuracy is up to nanometer level.