Abstract: Th is paper stud ies the expansive behav iour o f a new k ind o f expansive rock - brecc iatedm arl and fac to rs that influences the behaviour. A fter th is kind of rock is disturbed, the rock can absorb w ater and occur expan sion. It can br ing its' structure de fo rmation and damage even. The expensive force index is studied in the ba lancepressure method and the fu ll inflat ionmethod w ith GDG h igh pressure instrumen.t The rock samp le is laterally con fined. The paper po ints out the reason that the index of the full inf la tionm ethod is bigger than that o f the balance method. It stud ies the expansion law s and factors that influences expensive deformation w hen ample fu lw ater is sup plied to the brecciatedmarl in lateral restra int free expansion tes.t The rock samp le comes from Ta ihangshan tunne l of ShiTai Railw ay. The test result tells us that brecc iated marl is d ifferent from the o ther norm al expansive rocks. The ax ia l de fo rmation ratio increases as the rock w ater content ratio increases and reduces as the rock density in creases.