A head mounted display system comprises a lens set, an image capturing unit and a processing circuit. The lens set comprises a first liquid crystal panel and a second liquid crystal panel. The first liquid crystal panel comprises first liquid crystal blocks, and the second liquid crystal panel comprises second liquid crystal blocks. The image capturing unit captures front image data having a first dynamic range. The processing circuit performs tone mapping according to the front image data to generate mapping image data having a second dynamic range smaller than the first dynamic range. The processing circuit calculates regulated values according to the mapping image data. A driving circuit drives the first liquid crystal blocks and the second liquid crystal blocks according to the regulated values, respectively.