A magnetic field measuring device includes a support base, a measuring mechanism, and a displacement sensor. The measuring mechanism is positioned on the support base, and includes a mounting plate, a swing arm, a connecting member, an elastic member, and a measuring magnet. The mounting plate is connected to the support base. The swing arm is rotatably positioned on the mounting plate. The elastic member is positioned between the connecting member and the swing arm. The measuring magnet is connected to the swing arm, opposite to the support base. The displacement sensor is positioned on the support base, opposite to the swing arm.