A hat and a hat brim length adjusting method are provided to solve the problem that the hat brim length of the existing sunscreen hats cannot be adjusted in real time. The present hat includes a hat body and a hat brim which is retractable in length; the hat further includes a power supply, an adjustment unit and a detection unit; wherein the detection unit is used for detecting relations between a position of the sun and a position of a wearer's face in real time; and the adjustment unit is used for adjusting a length of the hat brim according to the detection result. The hat brim length of the present hat is adjustable, so that the hat not only can block the sun for the wearer but also can provide a good sight angle for the wearer.