With the rapid development of high voltage direct current (HVDC) transmission technology, it is necessary to assess the reliability of converter valve under different working conditions. In this paper, the working state of 6-pulse converter valve is divided into commutation period and non-commutation period. Two-phase short circuit is brought by commutation period and over-voltage impact will bring severe damages to the circuit especially under The influence of HEMP on the operation of the converter valve is studied. HEMP is also called high altitude nuclear electromagnetic pulse and it contains harmonics with different frequencies. HEMP will bring huge shocks to not only sources but valve. Lightning over-voltage is caused by thunder climate change and the high value of over-voltage may destroy the structure of thyristor. Operating over-voltage is brought by Therefore, in order to ensure the reliability and the operating efficiency of converter valve, we should focus on the influence of HEMP on the converter valve including the over-voltage distributions inside the HVDC converter valve and its impact to protecting circuits of thyristors.