An antenna system for a GPS receiver that includes a casing and a micro processor. The antenna system includes at least two antenna units, a micro controller, a switching unit and a signal processor. The antenna units are used for receiving a satellite signal. Each of the antenna units owns a signal-receiving direction for guiding the satellite signal. The micro controller determines an optimal signal-receiving direction for receiving the satellite signal according to a use mode and generates a control signal for choosing a specific antenna from the antenna units. The switching unit is coupled to the antenna units and can define a receiving route for enabling the specific antenna to receive the satellite signal according to the control signal.