The invention provides a slide rail installing mechanism comprising a rail member and an adjusting member. The rail member is provided with an upper all, a lower wall and a middle wall. The middle wall is connected between the upper wall and the lower wall. An accommodating groove is arranged between the upper wall and the middle wall. The adjusting member is flexibly installed on the rail member. The adjusting member is provided with an upper part and a lower part. The upper part is adjacent to the upper wall of the rail member, and the lower part is adjacent to the lower wall of the rail member. When the adjusting member is adjusted to move to a position, the adjusting member can stretch to the accommodating groove. The slide rail installing mechanism is suitable for bearing objects of different length, and the position of the adjusting member can be adjusted according to the demand without changing the structure of the rail member so that the adjusting member is enabled to block the installing member of the bearing object in the accommodating groove of the rail member.