Abstract: This paper introduced effective stress law under pore methane pressure. Test results prove the effectivestress law is in accord with the revised Terzaghi formula. The equivalent coefficient of pore pressure(a)is a bilinearfunction of volulmetric stress(Θ)and pore pressure(p).Based on the numerical features of a,4 zones were dividedin Θ-p area,i.e. the non-effective zone,normal effective zone,fracturing zone and the equivalent soil zone.