It is possible to set a special packer within the long completion horizontal interval to establish an injection zone and a production zone, and a new flooding scheme of simultaneous injection and production in a single-hole horizontal well drilled for developing thin marginal heavy oil reservoir or small block reservoirs or offshore reservoirs. In field practice, the fluids injection rate can be controlled with either concentric or parallel tubing strings. Reservoir numerical simulation was used to determine the formation thickness lower limitation for different viscosity reservoir and the optimum time to start steam flooding after steam soak by economic oil/steam ratio. The optimum time to start steam flooding is at the 7th cycle. There exists a peak recovery efficiency of steam flooding when the length of separation section ratio is 0.15-0.2. Oil recovery rate can be enhanced by increasing steam injection rate, but pressure difference also increase and the pressure difference increases sharply, and separation section by packer will not be in a stable state in the steam flooding process. A steam injection rate of 2.4t/(d.ha.h) was suitable for steam flooding under practical injection-production conditions. All the results could be useful for the guidance of steam flooding project.