The composition and structure of a stroke doubling device were introduced,and the fault types of the device in actual use were also described briefly.It was discovered that two chains’ uneven stress was the cause of all faults according to force analysis.Based on the analysis result,a load balancing mechanism using springs was designed to make sure that two chains supplied equivalent force.The particular structure of the load balancing mechanism and determination method of the sprin〖BF〗g’s〖BFQ〗 rigidity were expounded in detail.There is no fault anymore in actual use after the improved design is employed,and the improvement behaved well.