The complexity and cost of system-on-chip (SoC) designs keep increasing every year, which has progressively led to more opportunities for 2.5D System-in-Package (SiP) design. While 2.5D integration technology offers advantages for heterogeneous chiplet-based systems, it also poses challenges of a more complex overall design flow with limited EDA tools support, physical design optimization issues, interposer floor-planning difficulties and complex system-level verification. Furthermore, accurate inter-chiplet connection modeling, chiplet characterization and top-level simulation activities are also needed for comprehensive verification of SiPs. To tackle these challenges, we share an automated chiplet-based codesign flow: built on the backbone with standard EDA design tools, an automatic SiP register transfer language (RTL) generator is developed for top-level SiP netlist generation; inter-chiplet connection routing with chip assembly router and parasitic extraction tools; interposer design and bumps placement with the foundry defined redistribution layer (RDL).