The present invention provides a telecom fraud detection system and method based on user privacy protection, which can provide a new way of detecting fraud through the corporation of a plurality of operator systems, whereby by referring to the known fraud data in the operator system, the possible fraud hidden in the other operator systems can be found. Through an LDA model and an MMD calculation mode, the traditional telecommunication fraud can be found, the changeful telecommunication fraud also can be found, and the detection accuracy is greatly improved. In addition, the differential privacy approach is applied to ensure that the data transferring and reading are limited among the operator systems when the fraud is detected, so as not to leak privacy records and data of some special user accounts.