The trailing edge of high pressure gas turbine blade in aeroengine is usually designed as thin as possible to achieve higher aerodynamic efficiency. However, as the inlet temperature of modern gas turbine is continuously increasing, thermal stress in a thin trailing edge will become much significant, resulting in possibilities of erosion and creep problems. To find a balance between these two conflicting goals, one method is the use of pressure-side cutback, which extends into the coolant slot to get film cooling and also achieves a thin trailing edge. Due to the interactions between mainstream and coolant flow, film cooling effect on trailing edge cutback is significantly affected by the vortex shedding downstream the cooling slot. To resolve the coherent flow structures and understand their role on film cooling effect on trailing-edge cutback, this paper implemented a Very Large Eddy Simulation (VLES) model into the solver ANSYS Fluent with user defined functions. By introducing a resolution control factor, the turbulence viscosity predicted by transient SST k-ω model was corrected and the VLES computations were realized in the whole computational region. With the VLES method, film cooling effectiveness distributions on trailing-edge cutback at three blowing ratios were computed and compared against the experimental data. The coherent unsteadiness in cutback region was visualized to reveal the mixing process between mainstream and coolant flow. The numerical accuracies between different unsteady prediction methods, i.e. URANS (Unsteady Reynolds Averaged Navier-Stokes), SAS (Scale-Adaptive Simulation), DES (Detached Eddy Simulation), DDES (Delayed-Detached Eddy Simulation), SBES (Stress-Blended Eddy Simulation), and VLES were compared with respect to the resolutions of cooling effect and vortex shedding. The results show that the periodic vortex shedding induced by the interactions between mainstream and coolant is the main factor that affecting the cooling performance on cutback. VLES method has a comparable accuracy in predicting the film cooling effect on trailing edge cutback with DDES and SBES approaching. In the detached shear layer, VLES method exhibits a good ability to resolve coherent unsteadiness caused by vortex shedding. Compared with URANS and SAS methods, the VLES method has a higher accuracy in resolving the periodic vortex shedding and film cooling effectiveness distributions, especially in low blowing ratio cases.