The invention provides an apparatus for monitoring faulted deflection of a main body structure at the two sides of a deformation joint, for monitoring displacement change of the main body structure at the two sides of the deformation joint in a vertical direction and a horizontal direction. The apparatus comprises a double-shaft inclination angle sensor, a first support, a second support, a sensor installation plate and a connection shaft, wherein the double-shaft inclination angle sensor is installed on the upper surface of the sensor installation plate; the first support and the second support are respectively installed on the main body structure at the two sides of the deformation joint; and the sensor installation plate is arranged on the connection shaft, and the connection shaft is connected with the first support. A mode of one double-shaft inclination angle sensor is employed, deflection of the main body structure at the two sides of the deformation joint in an X direction and a Z direction is monitored, the occupied space is small, and installation and maintenance are facilitated.