By using the method of continuous polarization interfering,we have measured birefringence dispersion curve and in tensity transmission curve of quartz between 300 nm and 800 nm. By changing the width of emergent slit,we find that the extreme amplitude of intensity transmission curve varies along with the emergent slit and appears a definite rule. When the slit is wider,the light intensity extremum gather. The width of slit will only affect the extreme amplitude,but not affect the corresponding wavelength. The main factor is the monochromaticity of the emergent light. Material absorb is the secondary.