The invention provides a method for mapping the cross-sectional shape and diameter of a yarn. The two ends of a measured yarn to be tested are fixedly connected to a force measuring head on a tensiongauge and a slider, and the yarn is just at a set draft ratio state by controlling the position of the slider. The tension of the yarn is tested and recorded in real time by a dynamometer. A laser measuring sensor is rotated in the circumferential direction around the yarn, the cross-sectional shape of the yarn is drawn according to collected data, and the equivalent diameter of the yarn is calculated. The intensive point collection in the entire diameter direction is carried out in the measurement of the yarn, compared with an existing two-way measuring method in which a cross section is approximately circular, the method has the advantages that a result is more representative of the actual specific cross-sectional characteristics of the yarn, the mapped cross section is more accurate, anobtained diameter value is closer to an actual situation, the laser sensor is used to collect the data of the whole circumferential direction of the yarn, compared with existing CCD camera photographand image processing with an approximately circular cross section, the error is smaller, and the result is more accurate.