Numerical simulations are performed to obtain the water flow fields around a porous square artificial reef under the five different incoming velocities (0.1~0.8m/s). The numerical results of velocity distribution on the perpendicular flow plane passing through the center of the reef show that (1) under the five different incoming velocities the maximum and average velocities of the upwelling are 0.74 and 0.12 times of the incoming velocity, respectively, the maximum height of the upwelling is about 2.6 to 2.7 times of the reef height,and the length and height of the vortex region are 4 and 1.25 times of the reef height respectively; (2) a region where the flow direction is the same as that of the incoming flow is formed near the leeward surface of the reef. The numerical results of the velocity distribution on the horizontal flow plane passing the center of the reef reveal that (1) the length and width of the wake vortex region are 4 and 1.8 times of the reef width, respectively, for each of the five different incoming velocities; (2) vortex regions are also formed inside the reef. In this study, the similarities and differences with respect to the flow characteristics are also summarized preliminarily between the flow past a square reef without holes and the flow past a porous square reef.