In the light of energy balance and screw dislocation formation model,a detailed analysis is presented on strain relaxation mechanism of pseudomorphic SiGe based on the experimental result that shear modulus of low-temperature Si (LT-Si) is less than that of SiGe.The mechanism shows that strain is relaxed by dislocation formed in LT-Si buffer layer when the thickness of pseudomorphic SiGe film is smaller than the critical thickness, and dislocations prefecentially form in LT-Si layer then the thickness of the film is equal or exceeds the critical thickness,which agrees with the experimental results reported in the literature.At the same time,an experiment was carried out to grow relaxed Si0.8Ge0.2 virtual substrate using LT-Si technology.The results indicated that dislocations were resmicted to the LT-Si layer and the relaxation degree was 85.09% without threading dislocations in Si0.8Ge0.2.The experimental results proved that the strain of pseudomorphic Si0.8Ge0.2 is relaxed by dislocations formed in the LT-Si buffer layer.