Through more than 30 Km field survey in an important project of diversion water, there were many samples been taken from channel concrete lining and sluice pier to study failure mechanism of concrete due to sulfate attack at low value in soft water. The microstructure of concrete and mineral component of separated material was investigated by X-ray diffraction (XRD) and scanning electron microscope-energy dispersion X-ray analysis (SEM-EDAX). Then the processor of chemic action and physics action was analysis. Engineering situation shows that concrete can be failure quickly due to sulfate attack in soft water. Only one winter the strength of concrete reduced to zero. The test shows the failure mechanism is contact dissolution associating sulfate attack. The contact dissolution makes condition for sulfate attack. The concrete expand due to sulfate attack accelerate contact dissolution. So concrete can be failure in a short period.