The high-temperature pressure sensors have wide applications in aerospace, petroleum, geothermal exploration, automotive electronics, and other fields. However, the traditional silicon-based pressure sensors are restricted to pressure measurement under 120~{\circ }\text{C} and cannot be satisfied to measure the pressure of various gases or liquids in high temperature and other harsh environments. This article proposes a novel high-temperature pressure sensor based on graphene, in which a rectangular cavity is applied to improve the piezoresistive characteristics of the sensor. The unique of this sensor is that the graphene is coated by the silicon nitride (Si3N4) membrane, which could avoid the oxidation of graphene in high temperature and increase the temperature tolerance range. The sensor was placed at various temperatures ( 50~{\circ }\text{C} - 420~{\circ }\text{C} ) to explore the temperature characteristics, achieving a maximal temperature coefficient of resistance (TCR) of 0.322% {\circ }\text{C}{-{1}}. Moreover, the sensor with a 64 \times 9\,\,\mu \text{m}{{2}} cavity has a high pressure sensitivity of 5.32\times 10{-{4}} kPa {-{1}} , enabling a wide range from 100 kPa to 10 Pa. Experimental results indicate that the proposed sensor possesses superior pressure sensitivity, a wide pressure detection range, and a high-temperature tolerance of 420~{\circ }\text{C} , which provides new insight into fabricating high-temperature pressure sensors based on graphene and creates more applications in different fields.