The invention relates to a multi-machine PSS parameter optimizing and adjusting method suitable for a power grid multi-operation mode. The multi-machine PSS parameter optimizing and adjusting method suitable for the power grid multi-operation mode comprises the following steps: (1) calculating expectation and variance of characteristic values of all electromechanical oscillation modes in a systemby a standard daily operation curve for various node powder and PV node voltage in the system; (2) acquiring basic conditions for the system to have enough damping under the multi-operation mode on the premise that the characteristics of all the oscillation modes meet normal distribution assumed conditions; (3) carrying out PSS site selection through probabilistic eigenvalue sensitivity analysis;and (4) carrying out weight assignment on the different oscillation modes according to severity of the different oscillation modes to obtain a PSS parameter optimizing and adjusting objective functionand constraint conditions under the conditions that the severity of the different oscillation modes and multi-machine coordination are considered comprehensively. Robustness of PSS is improved, the circumstance that the stability of the system is reduced due to borrow damping phenomena is avoided, and the multi-machine PSS parameter optimizing and adjusting method suitable for the power grid multi-operation mode is suitable for carrying out coordination adjustment on PSS parameters in a multi-machine system which has multiple operation modes.