The urban space expansion is an important indicator for measuring urbanization level. Based on one Landsat MSS image and three Landsat TM images, the urban built-up characteristic of Hong Kong and its impact on the peripheral land during1973-2004 was analyzed. The urban area was 59.95km2 in 1973 and increased to 207.70km2 in 2004, the built-up area increased by 147.75km2 during the past 31 years and expanded 4.77km2 per year. The built-up area expanded around the old area and didn't present manifest change in a certain direction. The expansion rate was distinct from each other and Beijing city experienced two fast expansion stages comparatively. The increased built-up area originates from different land use type, of which about 33.28% converted from sea, and 13.34% converted from rural area and dependent factory, diggings land and other construction area. The other mainly came from arable that accounts for 39.93% of the whole added built-up area. Many factors have contributed to the urban land use changes. But four major driving forces, population growth, economic increase, transportation infrastructure improvement and institution and policy change, are the most important factors. Population growth can simulate the built-up area manifestly, the economic increase is the main impetus of the land use, and the transportation infrastructure improvement is the impetus for the urban expansion, while the institution and policy change determine the macroscopical development and expansion pattern in some degree.