An installation device for a slide assembly, comprising: a first rail; a second rail longitudinally movable relative to the first rail, the second rail having an opening, a first contact portion, a second contact portion and multiple holes including at least a first installation hole and a second installation hole, the first contact portion extending from an inner wall of the opening, the first installation hole located between the opening and the second contact portion; a retaining member having a base contacting the first contact portion of the second rail, a release portion extending from the base, a bent portion extending from the release portion and connected with an arm, the arm contacting the second rail so that the release portion is located away from the second rail; the arm extending from the bent portion and beyond the first installation hole of the second rail, the arm having a retaining hole and a positioning portion, the retaining hole located corresponding to the first installation hole of the second rail, the positioning portion located corresponding to the second contact portion of the second rail; a resilient portion extending from the arm and toward the release portion, the resilient portion connected to the second rail and providing a resilient force to keep the retaining hole of the arm facing the first installation hole of the second rail; the first installation hole of the second rail engaged with a first installation member of an exterior equipment, the second installation hole of the second rail engaged with a second installation member of the exterior equipment, the retaining hole of the arm of the retaining member accommodating the first installation member, the positioning portion of the arm contacting the second contact portion of the second rail, the first installation member located in the first installation hole by the retaining member; when a force is applied to the release portion, the arm of the retaining member being moved relative to the second rail and the positioning portion of the retaining member is disengaged from the second contact portion of the second rail, so that the first installation member is released from the first installation hole of the second rail and the retaining hole of the retaining member. a first rail; a second rail longitudinally movable relative to the first rail, the second rail having an opening, a first contact portion, a second contact portion and multiple holes including at least a first installation hole and a second installation hole, the first contact portion extending from an inner wall of the opening, the first installation hole located between the opening and the second contact portion; a retaining member having a base contacting the first contact portion of the second rail, a release portion extending from the base, a bent portion extending from the release portion and connected with an arm, the arm contacting the second rail so that the release portion is located away from the second rail; the arm extending from the bent portion and beyond the first installation hole of the second rail, the arm having a retaining hole and a positioning portion, the retaining hole located corresponding to the first installation hole of the second rail, the positioning portion located corresponding to the second contact portion of the second rail; a resilient portion extending from the arm and toward the release portion, the resilient portion connected to the second rail and providing a resilient force to keep the retaining hole of the arm facing the first installation hole of the second rail; the first installation hole of the second rail engaged with a first installation member of an exterior equipment, the second installation hole of the second rail engaged with a second installation member of the exterior equipment, the retaining hole of the arm of the retaining member accommodating the first installation member, the positioning portion of the arm contacting the second contact portion of the second rail, the first installation member located in the first installation hole by the retaining member; when a force is applied to the release portion, the arm of the retaining member being moved relative to the second rail and the positioning portion of the retaining member is disengaged from the second contact portion of the second rail, so that the first installation member is released from the first installation hole of the second rail and the retaining hole of the retaining member.