Through the summary of typical accidents，the problem between over speed protection devices of steam turbines of thermal power units and over frequency generator tripping measures is analyzed in isolated island operation of sending ends of the regional grid.The optimal allocation principle between over frequency generator tripping measures and over-speed protection equipment is put forward.The optimal scheme is given following analysis and comparison of a variety of programs.Typical models and parameters of domestic thermal power unit steam turbine over-speed protection have been collected for investigation.Typical models for thermal power unit steam turbine over-speed protection have been developed.Take Jiang-Zhong-Zhu power grid as an example，the whole process of isolated island operation is simulated.The coordination with currently available over frequency generator tripping measures and the relay setting related to power network is checked in isolated island operation，the validity and practicability of the optimal scheme and principle are analyzed.With the method，the regional grid will remain safe and stable.