In the OTA test of the antenna spatial performance of communication equipment,the total radiated power TRP and the total isotropic sensitivity TIS are two important characterization parameters,representing the overall transmission and reception performance of the antenna in space.In order to gain a deeper understanding of the physical meaning of these two parameters and the calculation method of TIS,this paper proposes a new method for deriving the TIS calculation formula.This method uses the reciprocity principle of the transmitting antenna and the receiving antenna.By comparing the transmitting process,two corresponding receiving antenna parameters are newly defined in the receiving process analysis.Combining the relationship between the antenna pattern and the receiving intensity,the relationship formula between TIS and equivalent isotropic sensitivity EIS is obtained after a simple deduction.This formula is consistent with the calculation formula given in the reference,but the derivation method in the reference is to use the ideal omnidirectional antenna as a reference,calculate the received power at the receiver input,and combine factors such as the actual antenna gain to derive the TIS calculation formula;the method proposed in this paper is to first derive the expression of the receiving directivity coefficient by definition,introduce the receiving strength parameter,calculate the full-space receiving power at the antenna,and derive the TIS calculation formula.In comparison,the derivation process in this paper calculates the received power from the input end of the receiving antenna,so there is no need to consider the efficiency of the antenna itself.The received power calculation starts directly from the receiving intensity.The entire derivation process is intuitive and easy to understand,avoiding complex concepts and formulas.