Abstract: The goaf is a great risk to work safely in open-pit mine，and roof is an important factor which affects the stability of goaf.The safety thickness of the proved goafs in Gongchangling open-pit mine was analyzed.The mechanical model of goaf roof under vehicle load was established，by which the vibrating equation of roof was deduced and the calculation formula of safety thickness were gained.Combined with actual situation in Gongchangling open-pit mine，the roof dynamic responses of vehicle speed，vehicle weight，thickness and span to the goaf roof beam were considered.And the minimum safe thickness of the roof with different tensile strength，span and different vehicle weight was calculated out.Based on this，the changing rule of safe thickness is analyzed.The results show that the dynamical bending moment has a linear relationship with the vehicle speed，vehicle weight and the roof span，and its calculated values decrease with the increase of roof thickness.The smaller the roof thickness is，the greater the dynamical bending moment decreasing is；The minimum safe thickness of the roof decreases with the increase of its tensile strength，while increases with the increase of span and vehicle weight.

Key words: Open-pit iron mine, Goaf roof, Vehicle load, Dynamical bending moment, Safety thickness