The engineering design of underground protective structures subjected to blast loading requires an appropriate stress-strain relationship for surrounding geomaterials. The behaviour of geomaterials under blast loading depends upon strain rate, stress level and interaction among the three phases. A few advanced constitutive models are proposed in the literature to model stress-strain behavior. However, a less accurate but simple alternative is to use functional forms for capturing the experimental stress-strain curves. In this paper, the functional form of stress-strain curve of geomaterials subjected to air-blast (uniaxial high strain-rate loading) is proposed based on the deformation mechanism of geomaterials. The proposed model consists of two different expressions for loading and unloading and requires only three parameters. The physical meaning of the three model parameters is discussed and the procedure for their evaluation is outlined. It is found that the proposed functional form captures the experimental stress-strain curves very well.