Taking a horizontal well as a uniform line source in a three dimensional space, this paper presents a mathematical model of horizontal well pressure drawdown and buildup in a single porosity reservoir and a double porosity reservoir by developing the necessary mathematical analysis. We first derive the point convergence pressure formulae, then according to the Superposition Principle of Potential, we apply quadrature theorem to derive pressure drawdown and buildup formulae of the uniform line source. Compared with the formulae published in the literature, our formulae, which do not include the sum of infinite series, are more reasonable and are easily used in well testing analysis.