Aiming at the cases of mal-operations of zero-sequence differential protection of converter transformers in the HVDC transmission project, the saturation mechanism of current transformers on the neutral line of the UHVDC converter transformer under long-term unipolar-earth operation accompanied with high-impedance fault conditions of AC system is analyzed. The reason of the mal-operations of the zero-sequence differential protection is revealed based on the UHVDC model, which adopts the actual engineering parameters. According to the polarity difference between the self-produced zero-sequence current and the zero-sequence current on the neutral line on both sides of the protection under internal and external faults, a novel blocking criterion of zero-sequence differential protection for converter transformers based on S-transform phase angle difference is proposed to solve the mal-operation of the zero-sequence differential protection under special conditions. The validity of the criterion is verified by simulation tests. This work is supported by National Natural Science Foundation of China (No. 51607106) and Open Foundation of Hubei Provincial Key Laboratory for Operation and Control of Cascaded Hydropower Station (China Three Gorges University) (No. 2018KJX05).