Abstract: In this paper，for the requirements of the manufacture of the terahertz coaxial multi-mode horn，synthesizing the accuracy of this horn are calculated with the Monte Carlo Method and the reasonable requirements of the tolerance for the horn are determined through the analysis on the performance of the horn such as return loss，max cross polarization level，equalization，etc. A terahertz coaxial multi-mode horn with the given tolerance is produced，and the test result of the horn is almost coincided with the simulation result，which verifies the validity of the tolerance design. Monte Carlo method can not only set tolerance in the given precision to meet the precision demand，but also avoid unnecessary precision waste.