An obstacle and cliff avoiding system is disclosed, which is substantially a movable chassis having at least a displacement sensor arranged thereon. In a preferred embodiment, each displacement sensor is tilted by a specific angle with respect to a surface detected thereby while being driven to move in circle by a rotating apparatus for enabling the same to scan a defined fan-like area or the circular area around the chassis in a circulating manner. As each displacement sensor is tilted by the specific angle and is initiated to issue a working signal, the paths of signal emission and reception of each sensor are defined by the specific angle, whereas the reflected working signal received by the sensor is sent to a control unit to be compared with the working signal so that the control unit is able to evaluate whether there is an obstacle or a drop in front of the moving direction of the movable chassis and thus control the chassis to maneuver around the obstacle or the drop.