Abstract: In the study of the transmission mechanism of electromagnetic wave in plasma in \varPhi 800 mm high temperature and low density shock tube in the Institute of Mechanics, CAS, under conditions of low density and strong shock, the experimental time at region 2 behind shock is significantly reduced due to the non-equilibrium processes such as gas dissociation and ionization. At the same time, the boundary layer effect leads to both the attenuation of the shock wave and the acceleration of the contact surface towards the shock front. Therefore, the experimental time at region 2 will be further reduced. These two effects lead to the reduction of the experimental observation time and the non-equilibrium state of test gas at region 2, resulting in the instability of data observation and the difficulty of data analysis. A mixture of argon and air is used to replace the pure air as the experimental test gas in \varPhi 800 mm shock tube. Since argon does not dissociate and is difficult to ionize, the compression ratio of shock is significantly reduced, thereby the test time and the gas length at region 2 are largely increased. The electron densities behind shock were measured with both the Langmuir electrostatic probe and the microwave transmission attenuation method. Meanwhile, the test times at region 2 were measured with the Langmuir probe. The results show that the electron densities in the mixtures of argon and air are in the same order of 10^13 cm^ - 3 as in the pure air. Under the same electron density and collision frequency conditions, the test times and the gas lengths at region 2 in two mixtures of 90%Ar+10%Air and 95%Ar+5%Air are about 5-10 times than those in the pure air. The test times at region 2 are about 300\sim800 \mu s, and the gas lengths at region 2 are about 1-1.5 meter. In electromagnetic wave transmission experiments in \varPhi 800 mm shock tube by using the argon and air mixture as the test gas, the results are more consistent with the theoretical prediction than those in the pure air.