Based on the traditional 4nterm model, an improved error model, which takes isolation and switch errors into account, according to actual composition of the hardware,is obtained and a method of error terms calibration is presented for multiport vector network analyzer (VNA). Then a measurement result correction algorithm is derived with the nport scattering parameters of DUT are expressed in form of matrix. Finally, a fourport actual device are measured in practice and the raw data is corrected with this algorithm. The final results are in good agreement with that of commercial VNA.