  Black widows (BWs) are millisecond pulsars ablating their companion stars.
The out-flowing material from the companion can block the radio emission of the
pulsar, resulting in eclipses. In this paper, we construct a model for the
radio eclipse by calculating the geometry of the bow shock between the winds of
the pulsar and companion, where the shock shapes the eclipsing medium but had
not been described in detail in previous works. The model is further used to
explain the variations of the flux density and dispersion measure (DM) of three
BW pulsars (i.e., PSR B1957$+$20, J2055$+$3829, and J2051$-$0827) detected by
the Five-hundred-meter Aperture Spherical radio Telescope (FAST). Consequently,
we constrained the parameters of the three BW systems such as the inclination
angles and true anomalies of the observer as well as the mass-loss rates and
wind velocity of the companion stars. With the help of these constraints, it is
expected that magnetic fields of companion stars and even masses of pulsars
could further be determined as some extra observation can be achieved in the
future.
