The invention discloses an optical fiber FP (Fabry-Perot) gas pressure sensor with optical vernier effect as well as a preparation method thereof. The sensor comprises single-mode optical fiber, a quartz capillary and photonic crystal optical fiber. The two ends of the quartz capillary are connected one end of the single-mode optical fiber and one end of the photonic crystal optical fiber correspondingly by a fusion welding mode; and the other end of the photonic crystal optical fiber is directly connected with the outside, and the pore structure facilitates the outside gas to enter a cavity of an optical fiber FP sensor, so that the quartz capillary and the photonic crystal optical fiber form two Fabry-Perot resonant cavities with vernier effect. During preparation, a commercial optical fiber fusion splicer is used, the discharging time and the discharging intensity of the electrode are controlled, two types of optical fiber are cascaded, a precise cutting device is used for cutting the optical fiber, and the length of the optical fiber is strictly controlled, so that the vernier effect is generated. The optical fiber FP gas pressure sensor is small in volume, simple and convenient to prepare, high in adaptability and high in sensitivity, and has wide application prospect.