An algorithm to generate unitary transformation (UT) of two orthogonal base kets is proposed in this paper. Certain requirements that UT must meet are as follows: four typical base kets in the first category can be transformed into states with the last qubit |0>, and the other four atypical base kets can be transformed into states with the last qubit |1>. This UT is applied to quantum data compression, with a result that the fidelity of the compression is 0.942. This method provides an important basis for realizing quantum compression and decompression. And it can be an important reference of other UT generation method which must fulfill some requirements.