An installation device for a slide assembly, comprising: a first rail; a second rail slidably connected to the first rail, the second rail having a hole, a guide seat, and at least one first slot and a second slot; a positioning member having a base, an arm board and an end board, the base fixed to the second rail, the arm board connected between the base and the end board, the arm board having a positioning hole located corresponding to the first slot of the second rail, the end board having a through hole and an end piece which extends from an end of the end board and toward the second rail and bends an angle; a release member having a reception portion, a connection board and an operation portion, the reception portion having a recessed reception face and a protrusion extending from the reception face, the connection board connected between the reception portion and the operation portion, a guide hole defined between the connection board and the reception portion, a fixing member extending through the guide hole and installing the release member to the second rail, the reception portion of the release member extending into the hole of the second rail, and the end board of the positioning member contacting the reception face of the release member and the protrusion of the release member extending through the through hole of the end board, when the operation portion of the release member is moved by an exterior force, the reception portion of the release member moves along the hole of the second rail, the protrusion of the release member contacts the end piece of the end board to push the positioning hole of the arm board to be away from the second rail, the operation portion of the release member resiliently moves and contacts the guide seat of the second rail to position the release member. a first rail; a second rail slidably connected to the first rail, the second rail having a hole, a guide seat, and at least one first slot and a second slot; a positioning member having a base, an arm board and an end board, the base fixed to the second rail, the arm board connected between the base and the end board, the arm board having a positioning hole located corresponding to the first slot of the second rail, the end board having a through hole and an end piece which extends from an end of the end board and toward the second rail and bends an angle; a release member having a reception portion, a connection board and an operation portion, the reception portion having a recessed reception face and a protrusion extending from the reception face, the connection board connected between the reception portion and the operation portion, a guide hole defined between the connection board and the reception portion, a fixing member extending through the guide hole and installing the release member to the second rail, the reception portion of the release member extending into the hole of the second rail, and the end board of the positioning member contacting the reception face of the release member and the protrusion of the release member extending through the through hole of the end board, when the operation portion of the release member is moved by an exterior force, the reception portion of the release member moves along the hole of the second rail, the protrusion of the release member contacts the end piece of the end board to push the positioning hole of the arm board to be away from the second rail, the operation portion of the release member resiliently moves and contacts the guide seat of the second rail to position the release member.