A sunshade device for a vehicle includes a sunshade cloth that has two opposite lateral ends inserted respectively into two track rails, a fixed end fixed to a winding shaft assembly, and a pull end interconnecting the lateral ends oppositely of the fixed end. A cross bar is attached to the pull end and extends between the track rails. A tensioning unit includes a spring disposed within the cross bar, a buffer sleeve disposed around the spring, and two tension cords each having an intermediate section attached to one of two lateral ends of the sunshade cloth, a first end section connected to one end of the spring, and a second end section connected to the winding shaft assembly. The tension cords prevent the sunshade cloth from becoming slack.