This paper proposes an all-optical-fiber high-precision magnetic field sensor based on magnetic fluid coating and cooling tapering to improve the sensitivity of optical fiber sensors in magnetic field detection and realizes high-precision real-time monitoring of field intensity under a weak magnetic field. The intermittent cooling method in the tapering process enhances the quality of the interference spectrum conveniently and slows the air-hole collapse of the photonic crystal fiber. Because of its simple manufacturing process, strong maneuverability, high sensitivity, and low degree of loss, the sensor realizes real-time online detection in a high-sensitivity magnetic field. Finally, the influence of the temperature variation of the sensor is discussed. Experimental results show that the photonic crystal fiber obtains a good interference spectrum at a tapering length of 5.5 mm and waist diameter of 75 μm. The sensor exhibits a highly linear response to external magnetic fields ranging from 0 to 78 Oe (1 Oe =^79.578 A·m ^-1) with a sensitivity of 95 pm·Oe ^-1. The degree of linear fitting is approximately 98.31%.