Abstract: A large scale distributed temperature data acquisition system based on self defined bus was proposed.To guarantee the system's reliability, a design based on the double computer in cold reserve was adopted. From system point of view,the system is a master slave and two level distributed construction which is composed of a master computer and several slave computers,those are responsible for management and temperature measurement respectively.In order to extend the use of the data acquisition unit,a special bus was defined, several function models and matrix sample network structure were developed as well.At last,the temperature error of the system were analyzed,precision norm was given to it.The results of the practical test work indicate that the system has high reliability and complete function for operation,and it could fulfill the large scale on line test task perfectly.