  Lepton scattering is an established ideal tool for studying inner structure
of small particles such as nucleons as well as nuclei. As a future high energy
nuclear physics project, an Electron-ion collider in China (EicC) has been
proposed. It will be constructed based on an upgraded heavy-ion accelerator,
High Intensity heavy-ion Accelerator Facility (HIAF) which is currently under
construction, together with a new electron ring. The proposed collider will
provide highly polarized electrons (with a polarization of $\sim$80%) and
protons (with a polarization of $\sim$70%) with variable center of mass
energies from 15 to 20 GeV and the luminosity of (2-3) $\times$ 10$^{33}$
cm$^{-2}$ s$^{-1}$. Polarized deuterons and Helium-3, as well as unpolarized
ion beams from Carbon to Uranium, will be also available at the EicC.
  The main foci of the EicC will be precision measurements of the structure of
the nucleon in the sea quark region, including 3D tomography of nucleon; the
partonic structure of nuclei and the parton interaction with the nuclear
environment; the exotic states, especially those with heavy flavor quark
contents. In addition, issues fundamental to understanding the origin of mass
could be addressed by measurements of heavy quarkonia near-threshold production
at the EicC. In order to achieve the above-mentioned physics goals, a
hermetical detector system will be constructed with cutting-edge technologies.
  This document is the result of collective contributions and valuable inputs
from experts across the globe. The EicC physics program complements the ongoing
scientific programs at the Jefferson Laboratory and the future EIC project in
the United States. The success of this project will also advance both nuclear
and particle physics as well as accelerator and detector technology in China.
