This disclosure provides a sail assist device, which includes a wind direction sensor for measuring a wind direction, a ship position acquiring module for acquiring a latitude and a longitude of the ship, and a display module for calculating a layline based on the latitude and longitude of the ship, a latitude and a longitude of a target position of the ship, and the wind direction, with the target position being a reference, and displaying a target mark indicating the target, a ship mark indicating the ship, and the layline.