  In this paper, we report radio observations of the Galactic Center magnetar
PSR J1745-2900 at six epochs between June and October, 2014. These observations
were carried out using the new Shanghai Tian Ma Radio Telescope at a frequency
of 8.6 GHz. Both the flux density and integrated profile of PSR J1745-2900 show
dramatic changes from epoch to epoch showing that the pulsar was in its
"erratic" phase. On MJD 56836, the flux density of this magnetar was about 8.7
mJy, which was ten times large than that reported at the time of discovery,
enabling a single-pulse analysis. The emission is dominated by narrow "spiky"
pulses which follow a log-normal distribution in peak flux density. From 1913
pulses, we detected 53 pulses whose peak flux density is ten times greater than
that of the integrated profile. They are concentrated in pulse phase at the
peaks of the integrated profile. The pulse widths at the 50% level of these
bright pulses was between 0.2 to 0.9 deg, much narrower than that of integrated
profile (~12 deg). The observed pulse widths may be limited by interstellar
scattering. No clear correlation was found between the widths and peak flux
density of these pulses and no evidence was found for subpulse drifting.
Relatively strong spiky pulses are also detected in the other five epochs of
observation, showing the same properties as that detected in MJD 56836. These
strong spiky pulses cannot be classified as "giant" pulses but are more closely
related to normal pulse emission.
