  1SWASP\,J162117.36$+$441254.2 was originally classified as an EW-type binary
with a period of 0.20785\,days. However, it was detected to have undergone a
stellar outburst on June 3, 2016. Although the system was latter classified as
a cataclysmic variable (CV) and the event was attributed as a dwarf-nova
outburst, the physical reason is still unknown. This binary has been monitored
photometrically since April 19, 2016 and many light curves were obtained
before, during and after the outburst. Those light and color curves observed
before the outburst indicate that the system is a special CV. The white dwarf
is not accreting material from the secondary and there are no accretion disks
surrounding the white dwarf. By comparing the light curves obtained from April
19 to September 14, 2016, it was found that magnetic activity of the secondary
is associated with the outburst. We show strong evidence that the $L_1$ region
on the secondary was heavily spotted before and after the outburst and thus
quench the mass transfer, while the outburst is produced by a sudden mass
accretion of the white dwarf. These results suggest that J162117 is a good
astrophysical laboratory to study stellar magnetic activity and its influences
on CV mass transfer and mass accretion.
