With the rapid increase of the capacity of optical fiber transmission system, the mode division multiplexing (MDM) transmission system using few-mode fibers (FMFs) (which provides the multi-channel multiplexing, high efficiency of frequency spectrum, and low nonlinear effects) becomes a research focus to upgrade the capacity of the optical communication. In this paper, an analytical expression of bending loss for each high-order mode of parabolic-index FMFs is deduced based on the perturbation theory and verified by finite element method. Based on this expression, the influence of four key structure parameters of trench-assisted parabolic-index FMFs: i.e. the radius of fiber core, the distance between core and trench, the width of trench, and the refractive index difference of trench, on the bending loss performance are discussed in detail. It is found that, firstly, the sensitivity of the bending loss increases with the increase of mode order of FMFs. Secondly, the smaller the core radius, the smaller the bending loss of each mode-order is, since small core radius leads to a smaller effective mode area, which is beneficial for saving power leakage. Additionally, the effective mode area of LP02 mode is lower than that of LP21 mode, while the bending loss of LP02 mode is higher than that of LP21 mode, this observation is different from other mode-orders. Thirdly, an optimized distance between trench and core for each high-order mode is also investigated for obtaining minimum bending loss, which plays an important role in controlling the bending performance of FMFs. So the higher the mode-order, the smaller the optimized distance between core and trench is, and this observation could be used to optimize the bending loss of the fiber. With the increase of the distance between the core and trench, the effective mode area of high-order mode increases quickly at first, then it is approximately unaltered. The distance between the core and trench is a key factor that influences both the bending loss and the effective mode area of each mode. Finally, the bending loss of each mode decreases with the increase of the width of trench around the fiber core or the refractive index difference of trench. These results are helpful for understanding the mechanism of bending loss for FMFs and are of significance for designing and manufacturing of few-mode bend-insensitive fibers, especially for the optimization of the bending loss of specific high-order mode.