Abstract: 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.

Key words: gob-side entry retaining, mechanical analysis of the stability of key block, width of roadside backfill, stability coefficient, sensitive coefficient of influencing factors