Due to long service time of the gob-side entry retaining and large deformation of surrounding rock

it is very important to calculate reasonably the width of roadside backfill. Based on activity rules of overlying strata of the gob-side entry retaining

The paper establishes a mechanical model of key blocks and immediate roof and analyses the interaction mechanism between key blocks and surrounding rock around the gob-side entry retaining

derives calculating formula of the gob-side supporting resistance

and introduces stability coefficients K1

K2 of key blocks and sensitive coefficients of influencing factors. If K1>1

extrusion instability will emerge; If K2>1

sliding instability will emerge. By taking No. 512（5） working face in Xieyi coal mine in

Huainan as project background. The paper determines reasonable width of roadside backfill

calculates stability coefficients of key blocks

and obtains sensitive coefficients of influencing factors on the stability of key blocks. The case study shows that the mining height has great influence on the stability of key blocks

so applying the technique of the gob-side entry retaining to working face with large mining height is of significant difficulty.