The Lobster Eye Imager for Astronomy (LEIA), a pathfinder of the Wide-field X-ray Telescope of the Einstein Probe mission, was successfully launched onboard the SATech-01 satellite of the Chinese Academy of Sciences on 2022 July 27. In this paper, we introduce the design and on-ground test results of the LEIA instrument. Using state-of-the-art Micro-Pore Optics (MPO), a wide field of view of 346 square degrees (186 × 186) of the X-ray imager is realized. An optical assembly composed of 36 MPO chips is used to focus incident X-ray photons, and four large-format complementary metal-oxide semiconductor (CMOS) sensors, each of size 6 cm × 6 cm, are used as the focal plane detectors. The instrument has an angular resolution of 4'–8' (in terms of FWHM) for the central focal spot of the point-spread function, and an effective area of 2–3 cm2 at 1 keV in essentially all the directions within the field of view. The detection passband is 0.5–4 keV in soft X-rays and the sensitivity is 2–3  × 10−11 erg s−1 cm−2 (about 1 milliCrab) with a 1000 s observation. The total weight of LEIA is 56 kg and the power is 85 W. The satellite, with a design lifetime of 2 yr, operates in a Sun-synchronous orbit of 500 km with an orbital period of 95 minutes. LEIA is paving the way for future missions by verifying in flight the technologies of both novel focusing imaging optics and CMOS sensors for X-ray observation, and by optimizing the working setups of the instrumental parameters. In addition, LEIA is able to carry out scientific observations to find new transients and to monitor known sources in the soft X-ray band, albeit with limited useful observing time available.