Through the analysis of the problem that the gear declosing mechanism of the horizontalvertical convertible rolling mill is difficult to disengage, sometimes automatically disengages from work,and its binding sleeve wears serious etc,and combined with the hydraulic system, the cause of the problem is find . Then the hydraulic system of the mill is improved.The practical results show that the problem has obviously improved, maintenance manhours and spare parts cost is reduced, the production efficiency is improved. 