Abstract: Based on the application experience of joint debugging on the control system of Jiuquan-Hunan ?800 k V UHVDC project, the limited amplitude fitting method and its realization principle of parameter setting of the GAMMA controller, voltage controller and current controllerare clarified, the sensitivity of high voltage and low voltage for the voltage controller and mechanism of the appended current factor of the GAMMA controller are discussed, the internal mechanism of the adverse change of transformer taps under the pole-starting and valve-exiting is investigated, and the control characteristics and its analysis methods of UHVDC under such special conditions as starting/stopping a pole, exiting a valve on-line, the voltage fluctuation of the AC system and the electromagnetic interference between poles are deeply studied.The analysis method and research results are helpful for analyzing the control characteristics of UHVDC system under complex conditions, and have strong practicability and universality.