The low load reactive power optimization in HVDC/UHVDC system plays an important role in voltage regulation for converter station and nearby substations. The principle of low load reactive power optimization（LLRPO） function is described and two kinds of LLRPO function of HVDC/UHVDC with different modes of implementation in China Southern Power Grid（CSG） are proposed. The corresponding performance and characteristics of the two functions are analyzed. The actual data of reactive power injection and bus voltage sensitivity analysis of rectifier station and nearby substations show that LLRPO function could significantly reduce reactive power exchange between DC and AC system under low DC transmission level so as to improve the system voltage quality and provide an efficient alternative of voltage regulation. Application shows that the LLRPO function can improve the voltage in the process of HVDC/UHVDC startup/shutdown and the low-load operating condition. Finally， suggestions are made for improving the LLRPO function in practice. The work is supported by National Mega-projects of Science Research for the 11th Five-year Plan（No. 2006BAA02A30）.