  A panchromatic investigation of morphology for the early-type spiral galaxy
M81 is presented in this paper. We perform bulge-disk decomposition in M81
images at a total of 20 wavebands from FUV to NIR obtained with GALEX, Swift,
SDSS, WIYN, 2MASS, WISE, and Spitzer. Morphological parameters such as Sersic
index, effective radius, position angle, and axis ratio for the bulge and the
disk are thus derived at all of the wavebands, which enables quantifying the
morphological K-correction for M81 and makes it possible to reproduce images
for the bulge and the disk in the galaxy at any waveband. The morphology as a
function of wavelength appears as a variable-slope trend of the Sersic index
and the effective radius, in which the variations are steep at UV--optical and
shallow at optical--NIR bands; the position angle and the axis ratio keep
invariable at least at optical--NIR bands. It is worth noting that the Sersic
index for the bulge reaches about 4--5 at optical and NIR bands, but drops to
about 1 at UV bands. This difference brings forward a caveat that a classical
bulge is likely misidentified for a pseudo-bulge or no bulge at high redshifts
where galaxies are observed through rest-frame UV channels with optical
telescopes. The next work of this series is planned to study spatially resolved
SEDs for the bulge and the disk, respectively, and thereby explore stellar
population properties and star formation/quenching history for the the galaxy
composed of the subsystems.
