Abstract: This study aims to investigate the effect of the slot synthetic jet issued from the rear surface of a square cylinder on the statistical characteristics of the flow field. The synthetic jet Reynolds number was kept constant at Re_Uj=45. The flow field around the square cylinder was measured using the time-resolved particle image velocimetry (PIV) under different parameter combinations of the dimensionless excitation frequency f_e/f₀ and the dimensionless stroke length L₀/w. Results show that the synthetic jet can induce a new recirculation region near the rear surface of the square cylinder and change the wake topology, the distributions of Reynolds stresses, the velocity profiles as well as the drag coefficient. It is found that there is an optimized combination of the dimensionless excitation frequency and the dimensionless stroke length, with which the synthetic jet has the best drag reduction effect. Under the current experimental condition, a maximum drag reduction rate of 53.7% is obtained at (f_e/f₀, L₀/w)=(4.0, 36.3), and this case has a faster recovery of mean streamwise velocity deficit in the wake, a lower level of velocity fluctuation and a longer streamwise length of recirculation region.