The invention discloses a serial high-temperature gas-cooled reactor nuclear energy system and an operating method thereof. The serial high-temperature gas-cooled reactor nuclear energy system includes a plurality of high-temperature gas-cooled reactors and a serial gas-cooled reactor. The high-temperature gas-cooled reactor includes a first reactor pressure vessel comprising a first reaction chamber for accommodating the first fuel element; a second reactor pressure vessel comprising a second reaction chamber interconnected with the first reaction chamber, allowing the first spent fuel in the first reaction chamber to enter the second reaction chamber. The system of the present invention allows the spent fuel discharged from the high-temperature gas-cooled reactor to be directly reused as fuel for the serial gas-cooled reactor, thereby improving the utilization rate of nuclear fuel and reducing the cost of gas-cooled reactors, which is conducive to the promotion of industrialized application of high-temperature gas-cooled reactors.