Abstract: According to the uncertain external load function location problem of the canopy in the hydraulic powered support,based on the assumed elastic rock beam of the roof,the paper studied the deflection variation condition of the roof rock strata above the coal mining face,derived the function location of the external load on the can opy of the hydraulic powered support and provided the principle to determine the canopy front and rear ratio of the two leg shield powered support and the canopy length of the four-leg shield powered support.The results showed that resultant force ratio of the hydraulic powered support on the rational location of the canopy was 2. 67 ,the theor etical results and experience data would be the same basically and could be applied to the design of powered support.