Abstract: The coaxial beam test stand was used to calibrate the beam position monitors and the beam current monitors. The electric and magnetic field in the beam test stand with displaced inner conductor was analyzed in theory. The results show that, when the impedance of the test stand is not very small, the electromagnetic (EM) field in the test stand is significantly different to the EM field in the beam-pipe generated by the beam. The difference of the EM field can lead to non-negligible calibration errors of beam monitors such as B-dot monitors and resistive wall monitors. The influence on the B-dot beam position monitors predicted by the theory calculation agrees well with experimental results.