With the development of modern electron and information technology, the problem of how to improve the measurement accuracy of Sparameters of noninsertion devices has beoame more and more important for microwave engineers. Unknown through calibration technique of VNA is forming by combination SOLT method and TRL method, by which noninsertion devices can be measured highprecisionly at calibration plane. After comparing the advantages and disadvantages of conventional method, the model, the calibration process and the application of unknown though technique are given. Experimental results show that the method can solve the problem effectively, can remove the measurement errors of external adapter even the real characters of the adapter being unknown, can improve the measuring accuracy with good flexibility.