The hadron therapy system at Institute of Modern Physics will provide carbon ion beams for clinical treatment of in-depth tumors.The beam delivery system

which is used to transfer the beam to irradiation fields

is critical for the heavy ion cancer clinical trials.The tumor is uniformly irradiated.Primarily accelerated by the cyclotron

the heavy ion beam pours into the cooler storage ring main （CSRm） for accumulation and acceleration.The ion beam is extracted by resonance

before the beam is wansferred to the tumor through the transfer line.Based on the medical requirements and technical specifications to design and optimize the transfer line

a set of lens parameters is adopted to flexibly adjust the beam spot size.The feasibility of a scanning system is verified by simulating the beam scanning process.