  Q-slope issue, which is caused by the field dependent surface resistance,
puzzled people for a long time in SRF fields. In this paper, we related the
Q-slope with surface treatments; and proposed a surface-impurity model to
explain the field-dependent of surface resistance of SRF cavities. Eighteen
cavity-test results have been analyzed to examine the model. These cavities
were treated by different recipes: Nitrogen-doping; BCP and HF-rinsing; EP with
120{\deg}C baking; and EP without 120{\deg}C baking. The performance of these
cavities, which is normally represented by cavity quality factor versus
accelerating gradient or surface magnetic field curves (Q0 vs. Eacc or Q0 vs.
B), has included all types of Q-slope, such as Low-field Q-slope, Medium-field
Q-slope, and Anti-Q-slope. The data fittings are quite successful; the fitting
results will be shown. The model can be used to evaluate the effectiveness of
the surface treatments. At last, the paper discussed the way to build a high-Q
high-gradient SRF cavity.
