We demonstrate a polarization-maintaining (PM) photonic crystal fiber (PCF) based Sagnac interfero meter for downhole high pressure sensing application. The PM PCF serves as a direct pressure sensing probe. The sensor is transducer free and thus fundamentally enhances its long-term sensing stability. In addition, the PM PCF can be coiled into a small diameter to fulfill the compact size requirement of downhole application. A theoretical study of its loss and birefringence changes with different coiling diameters has been carried out. This bend-insensitive property of the fiber provides ease for sensor design and benefits practical application. The pressure sensitivities of the proposed sensor are 4.21 and $3.24\mathrm{nm}/\mathrm{MPa}$ at $\sim 1320$ and $\sim 1550\mathrm{nm}$ , respectively. High pressure measurement up to $20\mathrm{MPa}$ was achieved with our experiment. It shows both good linearity in response to applied pressure and good repeatability within the entire measurement range. The proposed pressure sensor exhibits low temperature cross sensitivity and high temperature sustainability. It functions well without any measurable degradation effects on sensitivity or linearity at a temperature as high as $293°\mathrm{C}$ . These characteristics make it a potentially ideal candidate for downhole pressure sensing.