This paper proposes an analytical model for calculating air-damping effect in a bulk micromachined 2D tilt mirror. The theory for air-damping effect in this tilt mirror is derived from the nonlinear Reynold's equation. The boundary conditions are assumed at the ambient pressure on the sidewall of the mirror cavity. The analytical solution is obtained with Green's function for a rectangle mirror, which provides the insight of different influence of design parameters on the air-damping effect. Due to the similarity of the air damping and heat diffusion equations, the air-damping effect for arbitrary shaped mirrors is numerically simulated with a thermal-analog model, where the harmonic tilt motion of the mirror is substituted by a heat source varying with time and distance from the tilt axis. Results obtained from the experiments on prototype 2D octagon and circular tilt mirrors show excellent agreement with this air-damping model. Therefore, this model provides a simple approach to accurately calculate the air-damping effect experienced by a bulk micromachined 2D tilt mirror.