An input method and a smart terminal device. After a processor in the smart terminal device determines, based on a first detection solution of a first infrared sensor, that a target object enters or leaves a first space range from a first plane, the processor starts or stops displaying a cursor based on distances measured by ultrasonic sensors; and after the processor determines, based on a second detection result of a second infrared sensor, that the target object enters or leaves a second space range from a second plane, the processor starts or stops performing a confirmation operation based on a location of the cursor.