A medical optical human body size measuring instrument comprises a base plate, a stand column, a drive motor, linear guide rails, a synchronous belt, synchronous pulleys, an encoder, limit switches, sliding blocks, an annular measuring mechanism, optical sensors, a displayer and a main control panel. In addition, the invention further discloses a measuring method of the measuring instrument. According to the method, a measured person stands on the base plate of the measuring instrument, the sliding blocks move from top to bottom, and at the same time, the annular measuring mechanism continuously measures the body surface of the measured person to obtain three-dimensional body outline information of the measured person; new three-dimensional values (neck circumference, waist circumference and waist-hip ratio), height, chest circumference and other figure information are extracted from the three-dimensional body outline information of the measured person through pattern recognition and other intelligent algorithms. The three-dimensional body outline information, the new three-dimensional values, the height, the chest circumference and other figure information obtained through the method have great guiding significance on health management in real life.