Abstract: In order to solve the problem which permeation radius calculation formula was rare and its application wasrelatively poor in engineering practice, on the basis of the related assumptions, according to Darcy fixed-rate, newformula of penetration radius was derived. In the derivation, the grouting holes were divided into the ones crossingand not crossing the rock stratum, the angle between the grouting hole and grouting rock stratum, and the viscositytime-varying of the slurry were taken into comprehensive consideration. The formula was applied for the first time to acoal mine in Ningxia, the formula had a good practical value through the engineering practice test. This research maybe a reference to permeation grouting design and quality evaluation.