Exemplified by the martensitic transformation from DO3 to 18R in Cu-14.2Al-4.3Ni alloy and according to the principle that invariant-habit-plane can be obtained by self-accommodation between variants with twin relationships, and on the basis of displacement vector, volume fractions of two variants with twin relationships in martensitic transformation, habit-plane indexes, and orientation relationships between martensite and austenite after phase transformation can be calculated. Because no additional rotation matrixes are needed to be considered and mirror symmetric operations are used, the calculation process is simple and the results are accurate.