The paper introduces the 12 error terms calibration model algorithm of vector network analyzer(VNA) based on the signal flow graph theory and the Mason formula, and proposes the full 2-port calibration method. The influence of the error caused by measuring non-insertable DUTs with the VNA to the measuring results is analyzed and a corresponding compensation algorithm is given. Meanwhile, this paper discusses how the transmission measurement error is affected respectively by hardware performance of the VNA, the reflection coefficient and transmission coefficient of the through calibration kit and the S parameters of the DUT. Finally, an algorithm which can eliminate the unknown through is presented.