To investigate the effect of the thickness of directional steel fiber layer on the flexural behavior of geopolymer composite beam， geopolymer composite beams with different steel fiber layer thicknesses were fabricated and three-point bending tests were carried out. The experimental results show that the fracture energy of the composite beam increases as the steel fiber layer thickness increases， whereas the flexural strength increases with the steel fiber layer thickness at the beginning and then descends when the steel fiber layer thickness reaches a certain value. To further explore the reason behind this phenomenon， a simplified bending capacity model is formulated for partial and full sectional steel fiber reinforced geopolymer composite beam， respectively. By introducing the anisotropy of the compressive strength of the directional steel fiber reinforced geopolymer into the model， and taking the influence coefficient （β_tu） of directional steel fiber tensile strength as variable， the flexural bending capacity and the optimum thickness of steel fiber layer of composite beam are calculated and analyzed. When β_tu is about 3， the calculated results are in good agreement with the experimental results.