By analyzing planar laser-induced fluorescence (PLIF) images, the key parameters such as flame surface density (Σ), flame brush thickness and turbulent combustion velocity in the turbulent flame can be obtained, and the three-dimensional (3D) flame structure can be reconstructed based on two-dimensional (2D) PLIF images and the key parameters. It is not clear, however, whether the 2D PLIF images can accurately reflect the 3D flame structure. In this study, the 2D OH distribution of methane/air turbulent premixed flames was measured using orthogonal PLIF in both horizontal plane (perpendicular to the flame propagation direction)and vertical plane (parallel to the flame propagation direction), and the Σ was calculated by analyzing the OH-PLIF images. The Σ in the two planes were obtained under various conditions, i.e., different exit velocities, different locations, and different equivalence ratios. The results showed that the Σ in the vertical plane was smaller than that in the horizontal plane under almost all the conditions, and the difference in Σ between the two planes is decided by the burner exit velocity, the location, and the equivalence ratio. This phenomenon shows that the 2D PLIF technique has some limitations in accurately reflecting the 3D flame structure.