The High Energy cosmic-Radiation Detection (HERD) facility is a flagship and landmark scientific experiment onboard China's Space Station, planned for operation starting around 2025 for about 10 years. The main instrument of HERD is a 3-D calorimeter (CALO) sensitive to incident gamma-rays and particles from five sides. With this design, the effective geometric factor of HERD is more than one order of magnitude larger than that of previous missions. CALO is made of about 7,500 cubes of LYSO crystals, corresponding to about 55 radiation lengths and 3 nuclear interaction lengths, respectively. The crystal signals are transferred by wavelength shifting fibers and read out by ISCMOS devices. Energy deposition in each crystal is then derived by summing up about 400 CMOS pixels and with necessary correction for light saturation. Both a low range ISCMOS and a high range one are required to meet the requirement of a large dynamic range of at least 10 million. The prototype of CALO has been tested successfully in November 2015 at CERN, which leads to an improved design of CALO. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-Non Commercial-No Derivatives 4.0 International License (CC BY-NC-ND 4.0).