In an Ultrafast Electron Diffraction (UED), high-brightness ultrafast electron beams are indispensable to capture critical ultrafast events on an atomic/molecular scale. For space-charge effects (SCE) dominated electron beams, the beam emittance increases significantly during propagation. Understanding the beam emittance evolution during its passage is critical for further improving the UED performance. To diagnose the in situ emittance of the beam at several certain positions, we use a multi-slit device with a low sampling rate to eliminate the SCE influence. Due to the fabrication technology limitation, only a few slits can be made, leading to a severe undersampling rate, creating challenges in reconstructing the original beam information. This paper introduces a method to reproduce beam from severely under-sampled data.