  Swift J1818.0-1607 discovered in early 2020 is not only the fifth magnetar
known with periodic radio pulsations but also the fastest rotating one.
Simultaneous 2.25 and 8.60 GHz observations of Swift J1818.0-1607 were carried
out with Shanghai Tian Ma Radio Telescope (TMRT) from MJD 58936 to 59092. The
spin-frequency $\nu$ and first-order derivative $\dot\nu$ of this magnetar were
obtained with piecewise fitting method because of its instable timing
properties. We found that the amplitude of short-term $\dot\nu$ fluctuations
decreased with time, and the long-term declining trend of $\nu$ discovered
previously continued in our observations. The best fitted long-term $\dot\nu$
were about $-2.25 \times 10^{-11}~s^{-2}$ using our observation data spanning
156 days. The derived characteristic age was about 522 yr, supporting the
recent viewpoint that this magnetar may be older than initially thought shortly
after its discovery. The flux density of this magnetar was increased at both
2.25 and 8.60 GHz during our observations, and its radio spectrum became
flatter at the same time. We also detected bright-quiet type emission mode
switching in Swift J1818.0-1607.
