Ultrafast infrared photodetectors have evoked widespread interest in emerging areas such as high-precision frequency comb spectroscopy. However, existing mid-wave infrared photodetectors with high detectivity operate at cryogenic conditions, with a slow response time in the microsecond range. Hence, we report a high-speed room-temperature mid-wave infrared HgCdTe photodetector based on graded bandgap structure. This study explores a n-on-p homojunction structure on epitaxial HgCdTe, which achieves a total response time of 1.33ns (750MHz) under zero bias voltage at 300K, which is faster than commercial uncooled MCT photovoltaic photodetectors and MWIR HgCdTe APDs under high reverse bias. The analysis based on one-dimensional equations shows that compositional grading in the absorber layer can form built-in electric field and the transport mechanism of carriers is changed, the model is confirmed by the comparisons of different graded HgCdTe photodetectors. Thereby, our work facilitates design of the high-speed HgCdTe MWIR detectors, and provides a promising method to optimize the ultrafast MWIR infrared photodetectors.