The utility model discloses a steam supply device of an ultra supercritical power generation unit shaft seal. The steam supply device of the ultra supercritical power generation unit shaft seal comprises an auxiliary steam header and a shaft seal steam pipeline, wherein the auxiliary steam header and the shaft seal steam pipeline are communicated through a steam supply pipeline, the steam supply pipeline is orderly provided with a manual valve and an electric cooling pressure reducer, a filter screen is arranged on a connecting position of the steam supply pipeline and the shaft seal steam pipeline, and an electric heat tracing is arranged on the steam supply pipeline from an outlet of the electric cooling pressure reducer to the front of the filter screen. The steam supply device of the ultra supercritical power generation unit shaft seal can prevent accumulated water in the steam supply pipeline from being brought to a steam turbine shaft seal by steam, and prevents a shaft seal and a steam turbine main shaft from locking.