A smart motion detection device with a determining method includes a memory, a processor, and a sensor array coupled to the memory and the processor. An image captured by the sensor array is processed by the processor. The sensor array is adapted to pre-store the image into the memory when the processor is operated in the sleep mode, and the pre-stored image is transmitted to the processor when the processor is operated in the wakeup mode. The sensor array includes a comparator adapted to generate an alarm signal for switching the processor from the sleep mode to the wakeup mode in accordance with a comparison result of the pre-stored image. The determining method includes the processor analyzing images captured by the sensor array when the sensor array is activated to capture the images, and the processor analyzing images pre-stored inside the memory when the sensor array is not activated.