Troops evacuated villagers from remote mountain regions in the east of the island and helped secure their homes as rains and surging waves battered the coast.
More than 2,000 people, many of them tourists, had already been evacuated from Taiwan's outlying islands.
The typhoon is set to make a direct hit on the east coast in early hours of Saturday before moving across central Taiwan to Fujian province in mainland China.
The young girl and her mother became the first casualties of the impending storm after they were swept out to sea in Taiwan's eastern Yilan county yesterday.
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The dead girl's twin was also missing in the same incident, while another nine-year-old girl was injured but survived.
"The group went to the beach but were swept out to sea by strong waves," a spokesman for the fire bureau in Yilan County told AFP.
"It was a mother, her twins, and a friend's daughter. The adult and her daughter had already lost their heartbeat when brought to shore. The other child was conscious.
Troops helped move residents from aboriginal villages in the eastern counties Yilan and Hualien on Friday afternoon as well as reinforcing their houses, which will bear the brunt of the storm.
Riverside aboriginal communities in New Taipei City were also due to be evacuated Friday evening.
Much of the island will be lashed by torrential rain and by this afternoon 140 millimetres had already fallen near Taoyuan City in the northwest.
Offices and schools were shut down today, mainly in the north and east.
Packing maximum wind speeds of 173 kilometres per hour near its centre, Soudelor was 280 kilometres southeast of Hualien county this evening.