The invention relates to the field of direct-current transmission, in particular to a direct-current transmission converter valve open phase step circuit and an open phase step current determination method. The circuit comprises a stray capacitor Cs, a converter transformer leakage inductor Lmu, a power supply Us, a valve arrester Ra, a saturation resistor SR, a valve damping capacitor Cd and a valve damping resistor Rd. The determination method comprises the following steps of modeling the saturation resistor SR, a thyristor T and the valve arrester Ra in a non-linear mode, confirming the energy transfer relation of the converter valve open phase step circuit, analyzing a converter valve open initial state, simplifying the open phase step circuit, confirming the function relationship between phase step current and open phase step circuit parameters, optimizing system conditions and improving phase step current. An open instant energy transfer process is analyzed, the mathematic relation between the phase step current and the system parameters is built, and accordingly the open phase step current determination method which can correct to the ampere level is formed.