Deformation and strength of plastic concrete were studied by two kinds of true triaxial compression test. Lateral pressures are constant in first kind of test，and they were compressive prepeak and expansive postpeak, and strength is effected by the lateral pressures. In the second kind of test, those are increasing with increase of axial pressure and the strength was 9 time those of the uniaxial compressive one. The failure mechanism of plastic concrete under true triaxial compressive stress is as follows：When axial stress applied, lateral deformation is compressive because of constraint, and it retarde the appearance of expansive deformation, enhance anti racking performance. When axial compressive deformation is large enough, and lateral compressive deformations chang to expansive, and the volume tend to the minimum, and the strength reaches the peak quickly, but axial stress reduces slowly because lateral constraints. While axial compressive deformation accelerates after peak under the uniaxial compressive tests, axial stress reduces quickly without lateral constraints.