An inflator including a box composed of a cover and a base, and a compressor unit installed in the box, wherein the cover has a board, a front wall and a rear wall opposite to the front wall, and the base has a board, a front wall aligned with the front wall of the cover, and a rear wall aligned with the rear wall of the cover, the front walls of the cover and the base defining multiple first slits to serve as an air entrance port, the rear walls of the cover and the base defining multiple second slits to serve as an air exit port, the compressor unit including a motor having a rotating shaft, one end of which is provided with a cooling fan, behind the front walls, for drawing outside air into the box to form an air current via the first slits, the motor defining two opposite openings and at least one downstream through hole on its housing; characterised in that: the cover is provided at its board with two opposite, upper fixing columns capable of engaging with an upper airflow-guiding member, while the base is provided at its board with two opposite, lower fixing columns, corresponding to the two upper fixing columns, capable of engaging with a lower airflow-guiding member, whereby the upper and lower airflow-guiding members firmly supports the motor's housing of the compressor unit, and furthermore, the air current generated by the cooling fan can be guided by the upper and lower airflow-guiding members to quickly enter the motor's housing via the two opposite openings and to flow out of the motor's housing via the downstream through hole; wherein the upper airflow-guiding member has an intercepting wall, two lateral walls (35, 36) extending from two opposite sides of the intercepting wall thereof towards the front wall of the cover, a base plate formed between one end of the intercepting wall and the lateral walls (35, 36), and a curved portion formed opposite to the base plate, an upper slant surface being formed between the intercepting wall and the base plate of the upper airflow-guiding member and facing towards the front wall of the cover, the upper airflow-guiding member capable of engaging with the upper fixing columns such that the base plate of the upper airflow-guiding member is brought in contact with the board of the cover, and an upper air guiding channel is defined between the intercepting wall, the two lateral walls (35, 36) of the upper airflow-guiding member, and the base plate of the upper airflow-guiding member; the lower airflow-guiding member has an intercepting wall, two lateral walls (45, 46) extending from two opposite sides of the intercepting wall thereof towards the front wall of the base, a base plate formed between one end of the intercepting wall and the lateral walls (45, 46) of the lower airflow-guiding member, and a curved portion formed opposite to the base plate thereof, a lower slant surface being formed between the intercepting wall and the base plate of the lower airflow-guiding member and facing towards the front wall of the base, the lower airflow-guiding member capable of engaging with the lower fixing columns such that the base plate of the lower airflow-guiding member is brought in contact with the board of the base, and a lower air guiding channel is defined between the intercepting wall, the two lateral walls (45, 46) of the lower airflow-guiding member, and the base plate of the lower airflow-guiding member; wherein the two openings of the motor are respectively located within the upper and lower air guiding channels, whereby the upper and lower air guiding channels can guide the air current generated by the cooling fan to quickly enter the motor's housing via the two openings, wherein the upper and lower slant surfaces of the upper and lower airflow-guiding members allows the air current to pass the two openings more easily; wherein a first reference line is defined at an outer surface of the front wall of the cover or the front wall of the base; a second reference line is defined at a back of the cooling fan in front of a front end of the motor; the two openings defines at a portion of a circumferential wall of the motor's housing, near the front end of the motor, and a central axis of one of the two openings, being parallel to the front wall of the cover or the front wall of the base, is defined as a third reference line; a central axis of the two upper fixing columns or the two lower fixing columns, being parallel to the front wall of the cover or the front wall of the base, is defined as a fourth reference line; the distance between the first reference line and the fourth reference line is, the distance between the first reference line and the third reference line is, and the distance between the first reference line and the second reference line is, wherein the relationship of (A>B>C) is fulfilled; whereby outside air can be drawn into the box via the first slits and then be guided by the upper and lower airflow-guiding members to enter the motor's housing via the two openings, and then to flow out of the motor's housing via the downstream through hole, and finally to flow out of the box via the second slits, thus taking away the heat generated in the motor.