The invention discloses a preparation method for a specimen capable of being directly used for X-ray diffraction (XRD) detecting phase compositions. A V-shaped groove block is adopted for fabrication. A computerized numerical control wire-cut electric discharge machine is used for cutting. A stretch test machine is used for pulling off. Pressing process and sintering process are used for producing a sample piece, the computerized numerical control wire-cut electric discharge machine is used for producing cutting gaps, the sample piece is pulled off in a pulling-off mode and is enabled to be pulled to two pieces along a V-shaped groove and the cutting gaps, and therefore a pulling-off face is obtained. With the combination of the three technologies, the sample piece free from effect of stress and changes of phase compositions is obtained and can be directly used for XRD scanning. The preparation method for the specimen capable of being directly used for XRD detecting the phase compositions has the advantages of being precise in measuring data, simple in operation and convenient.