The early-time flow stage of a horizontal well is the flow period from the beginning of production to the moment when the nearest reservoir boundary is felt by the pressure wave created by the producing well. The early-time flow stage is described as the radial-flow period in the horizontal well literature. Several methods are available to calculate the end of this stage, but there exists significant differences among the results calculated by the methods in the literature, and the results lack bases in theory. Based on the productivity formulae of horizontal wells in Reference (1), this paper presents well testing analysis of horizontal wells in the early-time flow stage by instantaneous steady-state successive displacement method. Theoretic analyses and examples show that the formulae in this paper are accurate in describing the three dimensional percolation flow through porous media into horizontal wells in the early-time flow stage, and that the formulae are reasonable and reliable.