Chemical analysis of time-series vent fluid samples is needed to study the temporal changes in hydrothermal vent chemistry and surrounding biological communities. To meet this requirement, a gas-tight serial sampler was designed, constructed and tested. Based on the modular design principle, the serial sampler is currently composed of a control unit module and six sampling unit modules, which is convenient to be upgraded by adding more modules in future. Each sampling module consists of an independent sampling valve, a sample chamber and an accumulator. Compressed nitrogen gas is used to maintain the sample at in-situ pressure. The sampler is designed to be deployed 4000m under water. Laboratory experiment indicated a good sealing capability and pressure maintaining performance of each sampling module. In addition, the serial sampler was field tested during two sea trials, and gas-tight samples were successfully collected.