Pressure detection under high temperature environments has an urgent need in aerospace, equipment development, petrochemical and other fields. Especially, with the development of space industry and the research of new generation engines, in-situ pressure measurement of key parts such as pipes, chambers and combustion chambers plays an important role in engine test, combustion instability analysis, mode switching of engine systems and so on. Consequently, it is highly necessary to research on the design, manufacture and testing methods of the sensors. Compared with the traditional electrical sensors, fiber-optic sensor has the advantages of small size, long transmission distance and immunity to electromagnetic interference. Through the analysis of the high temperature pressure sensors based on fiber-optics Fabry-Perot principle, the operating temperature is mainly limited by the sensor composition material, sensitive unit processing method, signal transmission mode and demodulation method. The high-temperature resistance performance of sensitive materials directly determines the upper temperature limit of the sensor. Thermal stress mismatch is another significant factor causing the failure of sensors in harsh environments. In addition, the transmission and extraction methods of the characteristic signal are the difficult points of high-temperature pressure sensors. To address the above limitations, there are many researches on quartz, silicon, silicon carbide, sapphire and other high-temperature resistant materials as sensitive unit in high temperature region to improve the operating temperature. Meanwhile, MEMS and femtosecond laser processing methods have great advantages in sensor consistency and thermal stress matching manufacturing. Among the single-crystal oxide materials, single crystal magnesium oxide material shows high application value in the fields of high-temperature fiber-optic sensing because of its ultra-high melting point, excellent optical and mechanical properties. In this paper, combining the advantages of magnesium oxide material and MEMS processing technology, a magnesium oxide wafer optical fiber Fabry-Perot pressure sensor based on MEMS for harsh monitoring is proposed. The square Fabry-Perot cavity is designed to improve the pressure sensitivity. The thermal stress matching machining of the sensitive element is realized by developing the wet etching and direct bonding technology of magnesium oxide. Meanwhile, the optical fiber is integrated for pressure detection in high-temperature environments. The structural parameters of the sensor are optimized by the mechanical, thermal and modal simulation results. The pressure experiments at room temperature and high temperatures are carried out to verify the large pressure range and temperature response performance of the sensor. The experimental test results demonstrate that the FP cavity length of the sensor during the increasing and decreasing pressure over three cycles varies linearly with the pressure in the range of 15 MPa at room temperature with a nonlinear error of 0.75%FS. Additionally, error bars diagram is drawn to analyze the uncertainty and reliability of the sensor test results, which indicates that the sensor test results are relatively reliable. The high temperature pressure test results show that the sensor can be effectively measured in the range of 22 ℃ to 800 ℃. For each temperature, the FP cavity length decreases with the pressure, and the cavity length approximately linearly changes with the pressure over the entire test range, even up to 800 ℃. Therefore, the experiment results demonstrate that the sensor can stably operate at an environment of 22~800 ℃ and 0~0.7 MPa. This work is of fundamental importance in realization of pressure detecting in ultra-high environments. And the application of magnesium oxide in the field of optical sensing provides a new way to solve in-situ pressure measurement at high temperatures, narrow spaces or other related positions of aeroengines.