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Overvoltage is one of the key factors influencing whether Yunnan--Guangdong UHVDC Project can be operated in Islanded Mode or not. Based on the on site test parameters of primary equipments and the actual control and protection system of the project, this paper performs calculations of the overvoltage level in islanded operation models. The influence of BLOCK process and load retard speed of Yunnan--Guangdong UHVDC on the overvoltage level in islanded mode is also studied. The results show that it is an important measure for suppressing the overvoltage level to switch off the converter transformer after ac filters have been switched off. The maximum per unit switching overvoltage is 1, 71 (reference value is 449 kV) under various fault conditions. The maximum transient overvoltage is 1.66(reference value is 318 kV), but its duration is short, The maximum transient recovery voltage across AC filter/capacitor bank circuit breakers is less than 1 300 kV. The arrester's absorption energy in the case of a bipolar BLOCK can be greatly reduced with the protection logic modified with respect to decreasing current characteristic, which was compared with the protection logic of direct load retard. It is favorable for the reduction of both, overvoltage and arrester~ s absorption energy, to lower the force retard speed in the case of bipolar ESOF command.