This paper pointed out the shortcomings of other algorithms in the process of isoline smoothing after completing isoline tracking for the features of drawing rainfall isoline.The mathematical theory and merit of tension spline function in rainfall isoline smoothing were also introduced.The study shows that the tension spline function can adjust the coefficients automatically in terms of the node generating in the isoline tracking process,which resolves the intractable intersecting problem of the adjacent isoline smoothing.The rainfall isoline smoothed by the tension spline function algorithm in the process of making rainfall isoline map of Beijing Pinggu District has much better effects.