In this study, we developed a real-time GPU-based high frame rate (HFR) high dynamic range (HDR) imaging system at 500 fps from sequential 4 different low dynamic range (LDR) exposures which exposure time is controlled automatically by our high-speed virtual multi-thread exposure control module. Our automatic exposure (AE) control system is separated as 4 different threads virtually to simultaneously guarantee a best set of exposure times of the 4 sequential LDR images respectively at 125 fps. Based on the 4 different exposed LDR images, the HDR image is composited simultaneously in real time at 500 fps by introducing parallel image processing on a GPU board. The effectiveness of our system has been verified by performing experiment for high-speed motion under dynamically changing illumination.