A suspended-core fiber (SCF) with subwavelength core diameter is numerically analyzed for gas sensing application. The dependence of the relative sensitivity, effective mode area, and confinement loss on the fiber parameters as well as the refractive index of fiber material is investigated with finite element method. The simulation results show that the relative sensitivity and confinement loss will increase with the decrease of the core diameter and the fiber refractive index. The effective mode area decreases at first, and then increases when the fiber core decreases to a certain value. The confinement loss decreases dramatically with the increase of the air hole diameter, while the relative sensitivity and effective mode area remain unchanged. These results prove that the proposed SCF with subwavelength core diameter is very promising for developing a high-sensitivity gas sensor with large effective mode area and low confinement loss.