When HVDC transmission system operates in mono-polar condition or is struck by lightning current in the converter station, it will lead to unbalanced distribution of neutral current between YY and YD converter transformers. If the current difference is too large, it will cause damage to converter transformer and even HVDC block. In view of this technical problem, this paper reveals the mechanism of neutral point current difference in converter transformer based on transformer magnetic potential balance theory, and proposes a new method to restrain the neutral point current difference based on the construction of YY+d converter transformer with the third winding which connects to triangle.The PSCAD model based on the converter transformer parameters of actual project is built up for verification simulation. This paper provides a technical basis for the solution scheme for engineering problems caused by the current difference of converter neutral point, and has a reference and guiding significance for the design, operation, maintenance and protection configuration of converter transformers in practical DC projects.