A magnetic field sensing apparatus and a sensing method are provided. The magnetic field sensing apparatus includes an anisotropic magnetoresistive (AMR) resistor, a current generator, and an arithmetic device. The AMR resistor is configured to provide a first resistance value according to a sensed magnetic field in a first magnetic field sensing phase and provide a second resistance value according to the sensed magnetic field in a second magnetic field sensing phase by a magnetized direction setting operation. The current generator provides a current based on a current direction to flow through two ends of the AMR resistor. The arithmetic device is configured to perform an arithmetic operation with respect to a first voltage difference and a second voltage difference generated according to the current respectively in the first magnetic field sensing phase and the second magnetic field sensing phase, and generate a magnetic field sensing voltage result accordingly.