In order to analysis the seismic resistance mechanism of Staff apartment block of Beichuan telecommunication bureau (a six-storey-high masonry building in Beichuan County, meizoseismal area of Wenchuan earthquake), which just suffered from a moderate damage in the earthquake, ambient vibration survey and numerical simulation of structural earthquake response are carried out. Based on the ambient vibration survey, the natural frequencies of the building are acquired after Wenchuan earthquake. The first natural frequency in transverse and longitudinal is 3.45Hz and 3.65Hz, respectively, and the torsion frequency is 4.05Hz. The second natural frequency of the building in transverse is 7.55Hz, and that in longitudinal is not tested out. With Abaqus software, the earthquake response is computed out by inputting Wolong earthquake wave recorded in Wenchuan earthquake. The results of numerical simulation illustrate that the maximum acceleration amplification factor of the top floor in transverse and longitudinal is 5.28 and 6.68 severally. The maximum inter-story drift in transverse and longitudinal is 1/359 and 1/274 respectively, which are all exceed the limit of inter-story drift of masonry structure.