The invention discloses a high sulfur content gas reservoir sulfur solubility online test apparatus and a test method thereof. The high sulfur content gas reservoir sulfur solubility online test apparatus comprises a dual injection system, a CS2 absorption elemental sulfur system, a stratum environment high temperature and high pressure simulating reaction system, a fluorescence sulfur determining system, a data acquisition system and a cleaning and tail gas treatment system. The test method comprises sulfur-containing gas sample preparation, vacuumizing preparation, stratum high temperature and high pressure environment simulating, elemental sulfur testing, total sulfur metering testing, total sulfur testing after a sulfur dissolution reaction, data acquisition, cleaning and tail gas treatment, and the like. According to the present invention, the solubility data of the gas elemental sulfur in the high temperature and high pressure high sulfur content gas reservoir can be safely, conveniently, accurately and efficiently determined, the foundation is established for the solving of the widespread sulfur deposition problem in the high sulfur content gas reservoir development, and the scientific data support is provided for the making of the reasonable high sulfur content gas reservoir development program.