High_resolution x_ray diffraction technology is applied to the measurement of la ttice parameters and the study of strain for Hg1-xCdxTe fi lms gr own by molecular beam epitaxy. The results show that there exist both perpendicu lar strain and shear strain in Hg1-xCdxTe films. Based on crystal elastic theory, strain states in Hg1-xCdxTe films are analyz ed and calculated, and the lattice parameters of 1-xCdxTe film s at the relaxation state are obtained. It is found that the relationship betwee n lattice parameters and compositions of Hg1-xCdxTe films agrees with Vegard's law, rather than Higgins formula proposed in earlier research. It is also found that the lattice parameters can be derived from the measured data of (224) plane spacing, and that the compositions of Hg1-xCdxTe films can also be evaluated by using Vegard's law with an error about 0 01.