In this paper, the geomagnetic storm on May 18, 1910 recorded by Lu-Kia-Pang Observatory near Shanghai that didn't corne from solar activity but came from the effects of plasma tail of Halley's comet (1910II) is explained. The difference between the time of Halley's comet ingress (transit) and storm-onset is interpreted. By means of the H
γ component figure of geomagnetic field, we try to analyse the structure of plasma tail of Halley's comet and also discuss the mechanism of geomagnetic storm.