Abstract: A mathematical model is built for cavity temperature and pressure in underground detonation after the completion of cavity growth.It incorporates gas permeation in porous media,heat radiation between gas and cavity-wall and heat conduction between gas and ambient media.A corresponding one-dimensional code on the basis of spherical symmetry finite volume method is developed. Catty pressure and temperature of an underground nuclear test in alluvium at Nevada Test Site are computed.Measurment data of cavity pressure are analyzed in detail.