An analytical approximation solution to the mathematic model for calculation of soil infiltrability with linear-source inflow is presented, based on the mass-balance principle that when the supplied flow rate is constant. A cumulative infiltration equivalent to the Lewis–Milne equation is produced by ignoring water ponding at the soil surface. Measurement of linear source infiltration was conducted in lab and soil infiltration process was worked out using the numerical algorithm and approximate analytical algorithm. The calculated results were compared with that measured with the double-ring method, and meanwhile based on the principle of mass balance, relative deviations of the calculation models were worked out through comparing the volume of water infiltration with the actual water supply. It was 7.21%~14.8% for numerical solution and 3.31%~12.4% for analytic solution. The significance of this research is that both soil infiltrability and temporal equation of cumulative infiltration can be obtained directly, thus providing an efficient tool for relevant researches on prediction of rainfall–runoff process, hydrologic models and designing and management of irrigation systems.