  The study on the dynamic evolution of young supernova remnants (SNRs) is an
important way to understand the density structure of the progenitor's
circumstellar medium. We have reported the acceleration or deceleration, proper
motion and brightness changes of 260 compact radio features in the second
youngest known SNR Cas A at 5\,GHz based on the VLA data of five epochs from
1987 to 2004. The radio expansion center locates at $\alpha(1950)=23^{\rm
h}21^{\rm m}9^{\rm s}_{\cdot}7 \pm 0^{\rm s}_{\cdot}29,
\delta(1950)=+58^{\circ}32^{\prime}25^{\prime\prime}_{\cdot}2 \pm
2^{\prime\prime}_{\cdot}2$. Three-quarters of the compact knots are
decelerating, this suggests that there are significant density fluctuations in
the stellar winds of the remnant's progenitor. We have verified that the
acceleration or deceleration of compact knots are not related with the
distribution of brightness. The brightening, fading, disappearing or new
appearing of compact radio features in Cas A suggests that the magnetic field
in the remnant is changing rapidly.
