Experiments were conducted on a planner cascade test rig for the typical high pressure turbine blade profile VK ILS89 under conditions at a 0° inlet flow angle and exit Mach numbers of 0.55, 0.85, and 1.1. A high speed schlieren system was used to capture time-varying grayscale images of the region near the trailing edge of the blade. The variations in grayscale within these images are primarily caused by changes in the density gradient of the flow field and are correlated with the unsteady characteristics of the flow structure. Firstly, the mean grayscale value and roof mean square value of the time-series images were analyzed, with these values being correlated with the wake width and wake pulsation amplitude, respectively. As the exit Mach number of the blade cascade increases, both the wake width and wake pulsation amplitude increase, and a trend that is more pronounced at supersonic exit Mach numbers. In addition, the position of the wake centerline of the blade remains unchanged under subsonic conditions at the cascade outlet, but shifts towards the suction surface of the blade under transonic conditions. Further, the modal analysis results of proper orthogonal decomposition also reveal that the unsteadiness of the flow within the cascade is dominated by the wake under subsonic outflow conditions and by the wake-shock wave interaction under transonic outflow conditions.