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Swift J1818.0-1607 discovered in early 2020 is not only the fifth magnetarknown with periodic radio pulsations but also the fastest rotating one.Simultaneous 2.25 and 8.60 GHz observations of Swift J1818.0-1607 were carriedout with Shanghai Tian Ma Radio Telescope (TMRT) from MJD 58936 to 59092. Thespin-frequency $\nu$ and first-order derivative $\dot\nu$ of this magnetar wereobtained with piecewise fitting method because of its instable timingproperties. We found that the amplitude of short-term $\dot\nu$ fluctuationsdecreased with time, and the long-term declining trend of $\nu$ discoveredpreviously continued in our observations. The best fitted long-term $\dot\nu$were about $-2.25 \times 10^{-11}~s^{-2}$ using our observation data spanning156 days. The derived characteristic age was about 522 yr, supporting therecent viewpoint that this magnetar may be older than initially thought shortlyafter its discovery. The flux density of this magnetar was increased at both2.25 and 8.60 GHz during our observations, and its radio spectrum becameflatter at the same time. We also detected bright-quiet type emission modeswitching in Swift J1818.0-1607.

Simultaneous 2.25/8.60 GHz observations of the newly discovered magnetar -- Swift J1818.0-1607