Abstract: A large-scale view of the magnetospheric cusp is expected to be obtained by the Soft X-ray Imager (SXI) onboard the Solar wind Magnetosphere Ionosphere Link Explorer (SMILE). However, it is challenging to trace the three-dimensional cusp boundary from a two-dimensional X-ray image because the detected X-ray signals will be integrated along the line of sight. In this work, a global magnetohydrodynamic code was used to simulate the X-ray images and photon count images, assuming an interplanetary magnetic field with a pure B_z component. The assumption of an elliptic cusp boundary at a given altitude was used to trace the equatorward and poleward boundaries of the cusp from a simulated X-ray image. The average discrepancy was less than 0.1 R_E. To reduce the influence of instrument effects and cosmic X-ray backgrounds, image denoising was considered before applying the method above to SXI photon count images. The cusp boundaries were reasonably reconstructed from the noisy X-ray image.