In order to analyze the impacts that speed governor system has on the primary frequency regulation in isolated power grid, based on the frequency response characteristic, evaluation index for primary frequency regulation is proposed. And the influence on the primary frequency regulation by governor static difference coefficient, the ratio of generator participating in primary frequency regulation, the generator inertia time constant, and the governor time constant is analyzed by using root locus and predominant pole. The function of dead band, first frequency modulation limit and OPC (over-speed protection control) is argued. Meanwhile, based on static frequency characteristic curves, and methods for quantitative parameter adjustment are proposed. This work is supported by National Key Basic Research Program of China (973 Program) (No. 2012CB215200).