Single-photon acquisition is the key technology of free-space quantum key distribution. Single-photon acquisition probability is very important. Based Hermite-Gaussian beams in TEM20,TEM02,TEM30,TEM03 and TEM11,the theoretical model of single-photon acquisition probability is established. And the probability in different modes is shown by numerical simulation. The physical model of time filtering is established,the limited mode and optimal mode are analyzed. The results can provide theoretical basis for the design of single-photon source,and optimize the system of free-space quantum key distribution.