Abstract: The equivalent source reflection coefficient plays a crucial role in microwave power measurement and uncertainty evaluation. This paper first discusses the relationship between the source reflection coefficient in amplitude-stabilized signal source systems and the equivalent source reflection coefficient when measuring power ratios using three-port devices. The concept of "equivalent" in the equivalent source reflection coefficient is explained from the perspective of the mismatch factor. Next, the working principle and limitations of K. Shimaoka's method for measuring equivalent source reflection coefficient using a network analyzer and three-port devices are briefly introduced. Finally, experiments and comparisons are conducted using N-type power dividers and directional couplers as measurement objects, based on K. Shimaoka's method and the traditional formula method in the 1-18 GHz frequency range. Results indicate that K. Shimaoka's method for measuring equivalent source reflection coefficient has certain limitations. Due to the small difference in transmission coefficients obtained in this calculation method, it is sensitive to minor changes and thus unsuitable for measuring equivalent source reflection coefficients when using three-port devices with good directionality (such as directional couplers).