A low frequency oscillation incident caused by primal frequency modulation test on steam turbine generator units is described.The mechanism of oscillation is analyzed and the countermeasure is proposed.The basic oscillation characteristics are analyzed according to the low frequency oscillation waves recorded by the wide area measuring system，and the oscillation process is simulated in the PSCAD/EMTDC software.The principle of determining the low frequency oscillation mechanism（the negative damping or the resonance mechanism）is discussed.By referring to the damping characteristic curves of different steam turbine control modes and parameters，the cause of the low frequency oscillation incident is found to be the improper steam turbine control parameter setting which results in the negative system damping.To aviod this kind of low frequency oscillation，proper parameter setup of the steam turbine control system is essential.