  This paper presents the design and optimization of a LYSO crystal-based
electromagnetic calorimeter (ECAL) for the DarkSHINE experiment, which aims to
search for dark photon as potential dark force mediator. The ECAL design has
been meticulously evaluated through comprehensive simulations, focusing on
optimizing dimensions, material choices, and placement within the detector
array to enhance sensitivity in search for dark photon signatures while
balancing cost and performance. The concluded ECAL design, comprising
2.5$\times$2.5$\times$4 cm$^3$ LYSO crystals arranged in a
52.5$\times$52.5$\times$44 cm$^3$ structure, ensures high energy resolution and
effective energy containment. The study also explored the energy distribution
across different ECAL regions and established a dynamic range for energy
measurements, with a 4 GeV limit per crystal deemed sufficient. Additionally,
the radiation tolerance of ECAL components was assessed, confirming the
sustainability of LYSO crystals and radiation-resistant silicon sensors.
