As the Sijiaying iron ore rock has the characteristics of complete structure and of high rock drillability， medium-deep hole core drilling works and control the out-of-tolerance of drilling bending belong to the drilling technical problems and also the key to ensure the drilling quality. Since 2009 about 30 medium-deep holes has been constructed， two-thirds of them have been downgraded because of severe out-of-tolerance of the well deviation which confused the geologist as well as the drillers. In the process of drilling， some borehole accidents occurred， and deflected by screw drilling and eccentric wedge were failed to solve the problems. The formation rock is complete and hard， and the drilling deviation correction and building can not be realized， as a result， the severe out-of-tolerance of borehole bending in medium-deep holes has become an urgent technical problem which seriously affect the realization of the goal of "exploring the deep and blind area". In 2018， the design of drilling depth was 1500m， conventional drilling anti-inclination technique measures were adopted in order to ensure the drilling quality. What’s more， S75mm half-cycle self-correcting anti-bending drilling tool （XBZX） was tested， and achieved success to some extent compared to the construction in 2015， which provided technical support to solve the severe technical problem of bending out-of-tolerance drilling in medium-deep hole of Si-jiaying Iron Mine， and also provided practical experience and construction technology to prevent bending out-of-tolerance drilling in medium-deep hole of the same type of mining area.