Abstract: The compound soil-nailing wall was made of the soil-nailing wall and perpendicular protecting piles.The whole stability was checked in the design,while the pile length of perpendicular protecting piles affected directly the whole stability of the compound soil-nailing wall.There was a problem of a critical pile length in the perpendicular protecting piles.Setting out from an anti-sliding function of perpendicular protecting piles on the whole stability of the compound soil-nailing wall,the influence of the perpendicular protecting piles on the whole stability of the compound soil-nailing wall was analyzed,and some main factors of influence for the critical pile length were studied.In the engineering design,the perpendicular protecting piles should be designed according to the critical pile length.