intensity in the vicinity of the south-western footpoint had a slight enhancement. Fig.\[fig3\] (c) and (d) show the corresponding 304 $\rm\AA$ images observed by $SDO$/AIA instrument. It is difficult to distinguish any brightenings in the corresponding place where the brightening was found in H$\alpha$ band. This mean that this jet is H$\alpha$ jet instead of EUV jet. It is most likely to be caused by the different responding temperature between AIA 304 $\rm\AA$ band images and H$\alpha$ band images in the region with abundant plasma. In order to understand the property of the injected cool plasma, we make a time-distance diagram to estimate the velocity of the ejected cool plasma along the white dotted lines in the panels (a) and (b). Panel (e) shows the time-distance diagram derived by a series of the images from the NVST H$\alpha$. From this time-distance diagram, we distinguish four inclined lines. Based on their inclinations, we derive the projection velocities along the four dotted white lines, which are about 27.4 km/s, 37.8 km/s, 33.4 km/s, and 35.1 km/s, respectively. The mean projection velocity is about 33.4 km/s.

![The material injection event observed by the NVST. (a)-(b) H$\alpha$ images observed by NVST at 05:09:34 UT 