Taking advantage of the near-infrared (IR) absorption characteristics of gases, a sensor with an ultrasmall sample volume composed of a sealed slot waveguide and based on evanescent field absorption is proposed in this paper. Compared with a traditional open-slot waveguide, it features small volume antiparticles depositing pollution over the long-term and is insensitive to surroundings. Working at 1645 nm, a large evanescent field ratio of 0.27 is obtained by simulation and optimization; meanwhile, the propagation loss is around 1.6 dB/cm. The needed sample volume of the designed sensor under the structure parameters of w_air=40 nm, h_air=400 nm, and waveguide length=3 cm is approximately 480 μm3, which helps the sensor demonstrate excellent performance for gas analysis with an ultrasmall sample volume.