Phase transitions between the polytypes of boron nitride and the influence of defect and impurity on preparation of c-BN thin film are studied from energy and structure aspects. A pathway from h-BN to c-BN is analysed, namely: h-BN→r-BN→c-BN. The transformation from h-BN to c-BN is more difficult than that from r-BN to c-BN. The energy barrier is very high in direct transformation from h-BN to c-BN, but it is very low from r-BN to c-BN. In fact, defects and impurities in c-BN thin films may favorably drive the transformation from h-BN to c-BN. Defects and impurities can reduce the energy barrier for the transformation sufficiently enough for it to proceed under conditions obtainable in common laboratories. Based on the theoreticd model, we developed a new method to prepare c-BN thin film (three-step method). The effect of time and substrate bias voltage on the first step of preparation of c-BN thin film is investigated. The study proves that 5 min and -180V is very favorable. The c-BN thin film that content with cubic phase exceeding 80% can be repeatedly prepared using the three-step method. Results of experiments accord with the theoreticat model very well.