A discussion on the problems of calibration of test systems is presented. First，beginning from the definition of the test system and combining the specialties of the target of test systems，the features of test systems is analyzed. Then，aiming at the ambiguity on the calibration of test systems，such as synthetic calibration and system calibration，some definitions are discussed in detail. The characteristics of three basic calibration modes are identified and analyzed. Finally，to the difficulties on drawing calibration specifications of test systems，one scheme is taken out，which is the combination of different single parameter calibration specifications. And both the steps and process for synthetic calibration or system calibration are presented.