  The Crab Nebula is used by many instruments as a calibration source, in
particular at high energy, where it is one of the brightest celestial object.
The spectrometer INTEGRAL SPI (20 keV - 8 MeV), in operation since October
2002, offers a large dataset dedicated to this source, with regular campaigns
planned twice per year. We have analyzed the available data to quantify the
source behavior on a long term scale and examine the stability level on
timescales from hour to years. As a result, the source flux variability appears
to be contained within less than +/- 5% around a ~ 20 yr mean value, for broad
bands covering the 20 keV - 400 keV energy domain, above which statistic limits
any firm conclusion. In term of spectral shape, the Band model provides a good
description of the observed emission between 20 keV and 2.2 MeV. The averaged
spectrum best fit parameters correspond to a low energy slope of 1.99 +/- 0.01,
a high energy slope of -2.32 +/- 0.02 and a characteristic energy E c of 531
+/- 50 keV to describe the curvature joining both power laws. The spectral
parameters have then been determined on the revolution timescale (~ 1 to 2
days) and their steadiness confirms the source emission stability.
  As a complementary result, this study demonstrates that the SPI instrument
efficiency remains within 5% of its initial value, after 17 years of operation.
