Modular multi-level converter valve has the advantages of strong expansibility and fast response, etc., and has become the first choice for the construction of flexible HVDC transmission project. Its reliability is related to the safe operation of the whole transmission system, and the loss characteristic is related to the economic operation of the transmission system. In order to improve the reliability of flexible DC transmission system, the converter valve will generally set a certain degree of redundancy, but too high redundancy will bring serious losses. Based on the above considerations, this paper analyzes the relationship between redundancy and reliability and loss, and deduces its mathematical analytical expression. Then, a new optimization method of redundancy of MMC converter valve based on multi-objective optimization is proposed, which can make the HVDC system take into account the reliability and loss. Finally, the method proposed in this paper is verified by engineering examples, which shows that the proposed method can improve the comprehensive performance of the HVDC transmission system. This work is supported by National Key Research and Development Program of China (No. 2016YFB0901001).