The invention relates to a design method for setting PSS4B parameters based on frequency domain margin indexes to inhibit low-frequency oscillation, which is used for designing PSS4B parameters undermultiple running modes so as to better inhibit low-frequency and ultralow-frequency oscillation phenomena of a system and ensure that the system meets the requirement of stable running, and belongs tothe technical field of power grid safety. The design method comprises the following steps of: calculating singular values and left and right singular vectors by utilizing the rank-one characteristicof the controlled power grid transfer function matrix near the oscillation frequency, and performing truncated singular value decomposition; according to the truncated singular value decomposition ofthe controlled power grid, providing a target function for setting the multi-machine PSS4B parameter based on a frequency domain margin index; giving out a phase constraint condition of PSS4B in an ultra-low frequency band; determining the critical gain of PSS4B according to the diagonal dominance characteristic of the controlled power grid matrix in the excitation mode frequency band; and solvingthe optimization model to obtain the designed PSS4B parameter value.