Abstract: To avoid suddenly dropping during descending or ascending,the control means of the clamping machine in depot of primary department was modified. The clamper control was changed from PLC time delay sequential control to PLC sequential control combining with in-converter logic control. The accurate control of right clamping time was achieved by setting the control parameters of the frequency converter and modifying the relevant control program. The results of application showed that the improved clamping machine ran well, the incidence of unexpected drop decreased to 0 from 6 times/month, the equipment damage was effectively avoided,the maintenance cost was reduced by about 300 thousand RMB yuan per year,and the machine efficiency was increased.