Port crane is frequently used to load and unload cargoes in large mixed ship and hoisting equipment for ship manufacturing. Operation stability of the whole machine is largely determined by pros and cons of system performance of double link jib. Its optimization design has been the research focus in the industry, in the past, double link jib system is considered as a rigid link system to be designed, which ignores deflection change caused by its flexible deformation and excess energy consumption this may cause, however, double link jib system will be considered flexible deformation system in the paper, parameter optimization design is done for its main structure sizes, realizing the optimization of each lever's size in the jib system, cargoes could move approximately along the path of the horizontal direction when with payloads during luffing motion, so that luffing driving mechanism works with more energy-efficient and more smoothly. This study can make the construction of four-link mechanism more reasonable, more safe and reliable for the whole machine, which is a design method of green optimization in loading and unloading machine field.