A high-speed room-temperature mid-wave infrared HgCdTe photodetector based on graded bandgap structure was reported. This study explores a n-on-p homojunction structure on epitaxial HgCdTe， which achieves a total response time of 1.33 ns （750 MHz） under zero bias voltage at 300 K， 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， this work facilitates design of the high-speed HgCdTe MWIR detectors， and provides a promising method to optimize the ultrafast MWIR infrared photodetectors.