A method of classifying a mental state of a user using a computing device, which includes a microphone, a camera, a display, a wireless communication unit, and a processor, and a mental state classification server is provided. The method comprises: by the microphone, detecting ambient noise of the user; by the camera, generating an image by photographing a face of the user; by the processor, checking whether each of ambient noise of the user, ambient brightness of the face of the user obtained from the image, and a face position of the user obtained from the image is suitable for a heart rate variability measurement environment; by the processor, controlling to display on the display images indicating whether each of the ambient noise, the ambient brightness, and the face position is suitable for the heart rate variability.