A triple modular redundancy（TMR） emergency trip system（ETS） for steam turbine is presented in detail. The signal sampler， system bus， decision unit and output modular are designed in TMR structure to satisfy the high reliability requirements. All the system modules， with self-testing and diagnosis function， can be replaced online without affecting system operations. The host parameter synchronization scheme is designed to ensure the real-time and accuracy of system response. The adaptive voting scheme considering self-testing fault table of system modular is used to ensure the system reliability. The stability and reliability are attested by reliability analysis as well as field commissioning and operating.