Abstract: iscontinuity spacing in rockmass is very important for prediction of rock behaviour. The negative exponential probability distribution density of rock discontinuity spacing based on infinite length scanline measurement is the most important distribution function. A closed form function is proposed for correction of sampling base caused by finite length scanline in this paper. The result shows that the suggested method is quite effective in decreasing relative error percentage, this makes it possible to estimate discontinuity spacing of rockmase using short scanline. It is also of great importance for decreasing the measurement cost.