Stress strain relation of plastic concrete was studied using conventional triaxial and uniaxial compression tests with 4 plastic concrete mixes. The test results show that complete stress strain curve of plastic concrete in confining pressure is remarkably different from that in the uniaxial compression. There are longer unlinear transcending section, no distinct peak point, or sharply descending section. The relation is given between secant modulus and confining pressure, elastic modulus and strength under uniaxial compression stress. Relation is expressed between peak strain and confining pressure, strength and peak strain under uniaxial compression stress. At last, constitutive model of plastic concrete is presented in conventional triaxial stress state.