Abstract: In order to explore the expression that well describes the fluctuations of the section line of joints, ten typical JRC curves proposed by Barton are discretized by using the interval sampling, and then the discrete data points are fitted by different functions. The expression obtained from the fitting of the data is used to calculate the joint parameters of the standard JRC curve. Comparison between the results of the fitting and the calculated values of the joint parameters shows that the section line of the joint conforms to the trigonometric function. The approximate function obtained by the sine function approximation can well characterize the fluctuation of the joint. The values of JRC calculated by the approximate joint curve replaced by the sine approximate function are more accurate than those obtained by using the discrete points. This also makes it possible to accurately estimate the joint shear strength by using the JRC-JCS model.