Abstract: In order to get acquainted with the characteristics and changes of production by water flooding and to make predictions about its sweep efficiency, a field pilot test was conducted in 1965 in the Daqing oil field on a closespaced well pattern. The results of the test have been proved to be reliable by means of numerical simulation and core analysis. They are summarized in the following: 1. lower recovery-bbl/psi drop when the water cut is less than 60%, a steady output may be maintained if the reduction of the difference between bottom hole pressure and saturation pressure is gradual; 2. flowing production is suitable when the water cut is less than 60～70%; when water cut exceeds this limit, other means of production should be chosen on the basis of techno-economic analysis, and 3. the recovery factor by water drive in the Daqing oil field is 40%, the increase of water cut is rather rapid and the period of production with water cut is quite long.