The underwater radiation efficiency is an important measure to understand the noise radiation characteristics of underwater structures. In this paper, we compare the measurement characteristics of the near field method and the far field method, which measure underwater radiation efficiency, through simulations using a numerical model. An immersed spherical shell was modeled by using finite element / boundary element fluid-structure coupled analysis formulation and the response calculation process was verified by comparing with the analytical solution. Numerical simulations of the underwater radiation efficiency simulation show that the near field and the far field measurements in the low frequency band can overestimate the radiation efficiency and the error in the near field method can reach 10 dB. Therefore, in the measurement of the underwater radiation efficiency, it is necessary to consider the measurement characteristics in the experiment methods and select the test method through proper balance with the expected signal to noise ratio.