Abstract: Making the type of locking set of 65B drilling machine of the LTV company in U. S. A. a example, some problems about the locking set of expend and contract derrick hydraulic and with single mast were discussed, such as its use, designing requirements, basic structure, working principles, operating procedures and so on. The analysis about the power which the main part of this set suffers and the ways of calculating the strength of the main part were studied. The precision of making the locking set and the chink between the upper and lower parts of the derrick as well as the effects of the length of the connecting part of above two parts on the locking precision were also studied. Finally, a piece of advice was given, that is to change the mechanical operation of the locking set of 65B drilling machine of the LTV company of U. S. A. into hydraulic operation.