On the basis of engineering example analysis, the cause of unevenness in quality of dry jet mixing pile shaft was detailedly analysed and explained from reinforcement mechanism, construction technology and quality test respectively. The study results indicated that the strengths of dry jet mixing pile shaft either in vertical direction or in radial direction are uneven, and the effective depth in treating soft foundation by dry jet mixing method is limited to 10 meter. This result may offer reference for other projects.