In order to measure the tree diameter at breast height (DBH) rapidly, efficiently and accurately, and improve the work efficiency of forest resource investigation, a portable high precision equipment for measuring tree DBH was developed. This equipment was integrated by central processing unit (CPU）, DBH sensor, liquid crystal display (LCD), storage, wireless communication module and so on. The DBH sensor was developed independently by this experiment. The principle of the DBH sensor was to convert the measured length information into voltage signal by using electronic technology and mechanical principle. The voltage signal would be calculated by CPU and the measured value of DBH was obtained. Wireless communication was used to cooperate with the mobile phone that installed with the data receiving terminal APP. It can achieve the functions of DBH measurement, data encoding, data recording, data editing, data storage and data export. According to the results of the test data, the measurement accuracy of this equipment for different tree species reached more than 99.97%. This device can adapt to different kinds of trees and different diameter trees, which accorded with the national survey accuracy standard of continuous forest inventory. And the measuring efficiency of this equipment was more than twice higher than that of traditional diameter gauge.