The detailed simulation is presented to repeat the incident on the 7th July， 2006 in the Guiyang South Grid. The simulation is conducted by using NETOMAC based on the real network data， testing data of the field measurements， and supplement data of classical values， models of the real prime mover， governor， OPC and load of the China Southern Power Grid. Through further investigation， it shows that the improper limit setting of primary frequency regulation is the main cause of high frequency in the isolated sub-grid and proposes methods to damp the frequency fluctuation. Some issues of the frequency security problems of the grids at present are discussed， and it is pointed out that the frequency stability should be researched as soon as possible.