  We introduce a method of subtracting geocoronal Halpha emissions from the
spectra of LAMOST medium-resolution spectral survey of Galactic nebulae
(LAMOST-MRS-N). The flux ratios of the Halpha sky line to the adjacent OH
lambda6554 single line do not show a pattern or gradient distribution in a
plate. More interestingly, the ratio is well correlated to solar altitude,
which is the angle of the sun relative to the Earth's horizon. It is found that
the ratio decreases from 0.8 to 0.2 with the decreasing solar altitude from -17
to -73 degree. Based on this relation, which is described by a linear function,
we can construct the Halpha sky component and subtract it from the science
spectrum. This method has been applied to the LAMOST-MRS-N data, and the
contamination level of the Halpha sky to nebula is reduced from 40% to less
than 10%. The new generated spectra will significantly improve the accuracy of
the classifications and the measurements of physical parameters of Galactic
nebulae.
