Abstract: Analysis on the causes of frequent damage of large arms at the Joint between the large arm and the middle arm oil cylinder about a certain type of roadway repair machine .By analyzing the design method, processing process and working load.It is clear that the damage of the large arm is due to the calculation error of the working load during the design process,it causes lack of strength in large arms.By calculation,the pull of the middle arm cylinder to the large arm is 240kN when the bucket rod oil cylinder is used to collect the material,the pull of the middle arm cylinder to the large arm is 480kN when the bucket oil cylinder is used to collect the material,it is greater than the oil cylinder can withstand maximum tension when 196kN when the balancing valve is forced to open.Finally, the improvement measures of the large arm are put forward.