Abstract: The early flow of the horizontal well means the phase from the beginning of the production to the propagation of the pres-sure difference that did not reached the reservoir boundary yet.The phase was regarded as the radius flow in the published liter-ature, there are various formulations to calculate the early end time, the results of different equations have big errors, and
some of them lacked theoretical supports. According to the earlyiso-potential surface, that is the revolved ellipse surface with the two ends of the horizontal well, the early well testing equations and the calculating method of the early end time were derived in light of the mass conversation. The theory and practice have shown that the equation derived out in the paper more described the early fluid flow law, the calculated results were more rea-
sonable than those before.

Key words: horizontal well, early flow, well testing for-mulation