This paper uses sounding data, NCEP reanalysis data and observation data of X-band dual-polarization radar to analyze the hailstorms triggered by small troughs. According to the features of occurrence and development and the predictive indexes with good indicative significance to the hailstorm triggered by small trough, a diagnosis method is constructed. By the method, three hailstorms with different intensities triggered by small trough are diagnosed, and the prediction effects are verified. The results indicate that the hailstorm potential of individual cases can be diagnosed effectively by the thresholds of the vertical helicity of humid heat force greater than 0.8×10-3 Pa·m3·K·kg-1·s-2 and the vertical helicity of water vapor greater than 0.8×10-5 kg-1·m3·Pa·s-2 in the initial stage of hailstone development. Strong wind shear and cell merging processes promote the occurrence and development of hailstorm. In view of this feature, the probability of hailstorm cell developing into severe hailstorm can be diagnosed by using the correspondence between the large value center with the absolute value of thermo shear advection parameter greater than 3×10-8 K·Pa-1·s-1 and the threshold value of the relation between 45 dBz echo top height and 0℃ height. Finally, it is verified that this method can diagnose the hailfall potential and severe hailstorm process comprehensively.