In order to explore the effect of the synthetic jet through a spanwise slot on the friction drag reduction of the turbulent boundary layer of a flat plate, the streamwise velocity were measured by a constant temperature hot-wire anemometer with and without synthetic jet. According to the measured experiment data, the mean velocity profiles, fluctuating velocity profiles, skewness and flatness were obtained respectively. Typical combinations of different frequencies and intensities of synthetic jet were selected to study the variation of the forcing effect in the streamwise direction. The experimental results showed that the skin friction drag reduction in the turbulent boundary layer was related to the characteristics of the imposed synthetic jet and the distance between the local station and the slot. The synthetic jet led to an increase of the skin friction drag in the station close to the slot, and as the flow moved downstream away from the slot, a reduction in the skin friction occurred with the tendency of firstly increasing and then decreasing. The reduction of skin friction caused by the synthetic jet of a higher frequency was more than that of a lower frequency. The power spectral density and autocorrelation of the fluctuating velocities indicated that the effect of the synthetic jet on the turbulent boundary layer decayed in the streamwise direction.