Making use of the new model of falt-topped light beams proposed by Li recently,the analytical expressions of the M~2-factor and the field distribution of flat-topped light beams passing through a paraxial ABCD optical system have been derived,respectively.The lens system is taken as an example and the analytical expression of the focusing field distribution of flat-topped light beams is given.The focusing properties of flat-topped light bemas have been analyzed in detail.The results have shown that the M~2-factor is only decided by the order M,the focusing properties relate to the order M and the Fresnel number N of the system.