The Hard X-ray Imager (HXI) aboard the Advanced Space-based Solar Observatory (ASO-S) is an instrument designed to observe hard X-ray (HXR) spectra and images of solar flares. Having 91 subcollimators to modulate incident X-rays, HXI can obtain 91 modulation data and 45 visibilities to reconstruct images with a spatial resolution as high as $\sim 3.1$ arcsec. HXI was launched on 9 October 2022 and powered up on 17 October 2022. After the on-orbit testing phase lasting for three months, HXI was ready for regular observations on 18 January 2023. With fine-tuning of the detectors and electronics, we were able to expand the energy range from $\sim 30$ – 200 keV to $\sim 10$ keV – 300 keV, which significantly raised the scientific values of the data and the number of detected flare events. This paper presents the changes and improvements of HXI instrument since 2019, the important ground tests, on-orbit tests, and calibration works. We also present the light curves, spectra, and reconstructed images of one flare observed by HXI on 6 January 2023.
