The invention discloses a high temperature gas cooled reactor direct hydrogen production coupling steam cycle power generation system and a method thereof. The system comprises a high temperature gas cooled reactor, a conversion reactor, a steam generator, a coolant cycle pump, a steam superheater, a turbo generator set, a condenser, a water-feeding pump, a middle temperature converter, a steam extraction pipeline and a natural gas pipeline, wherein a helium outlet of the high temperature gas cooled reactor is connected with a helium inlet of the conversion reactor, a helium outlet of the conversion reactor is connected with the steam generator, a conversion gas outlet of the conversion reactor is connected with the steam superheater, a water steam outlet of the steam generator is connected with a steam inlet of the steam superheater, a steam outlet of the steam superheater is connected with a steam inlet of the turbo generator set, and a steam extraction port of the turbo generator set outputs a blast of steam and is connected with a raw material inlet of the conversion reactor. Through the system, a hydrogen-power cogeneration mode is realized, temperature matching, the high heat utilization rate, low hydrogen production cost and high steam cycle efficiency are realized, and the system further has excellent economic benefits and application prospects.