  Using the multi-instrument observations, we make the first simultaneous
imaging and spectral study on the null point of a fan-spine magnetic topology
during a solar flare. When magnetic reconnection occurs at the null point, the
fan-spine configuration brightens in the (extreme-)ultraviolet channels. In the
H$\alpha$ images, the fan-spine structure is partly filled and outlined by the
bi-directional material flows ejected from the reconnection site. The
extrapolated coronal magnetic field confirms the existence of the fan-spine
topology. Before and after the flare peak, the total velocity of the outflows
is estimated to be about 60 km s$^{-1}$. During the flare, the Si IV line
profile at the reconnection region is enhanced both in the blue-wing and
red-wing. At the flare peak time, the total velocity of the outflows is found
to be 144 km s$^{-1}$. Superposed on the Si IV profile, there are several deep
absorption lines with the blueshift of several tens of km s$^{-1}$. The reason
is inferred to be that the bright reconnection region observed in Si IV channel
is located under the cooler material appearing as dark features in the
H$\alpha$ line. The blueshifted absorption lines indicate the movement of the
cooler material toward the observer. The depth of the absorption lines also
depends on the amount of cooler material. These results imply that this kind of
spectral profiles can be used as a tool to diagnose the properties of cooler
material above reconnection site.
