[Objective] Through the investigation of the "7·25" massive mountain torrents geological hazards in Tianshui City of Gansu Province occured in 2013, we expected to reveal the landslide mechanism and provide a scientific basis for early identification, prevention, and reduction of rainfall-induced geo-hazards in the loess ridge-hill areas of China. [Methods] The characteristics, distribution law of geological hazards and its causes were studied. Three dimensional continuum dynamic numerical models were established to calculate the dynamic character of landslide in Tianshui City. [Results] The results showed "7·25" massive mountain torrents geological hazards had characteristics of occurrence universally, outbreak massively, locally and unnoticeably, and had strong hazard potentiality. The hazards were exacerbated by the effects of earthquake and rainfall. The slope geometry shapes of landslide in the torrent's geological hazard are linear concaved. Most slopes are in the range of 35°~45°, and among which, 40° slopes were found more frequently. Loess slope landslide is usually in a narrow strip and with small volume. The average velocity, maximum velocity, total kinetic energy and potential energy of typical landslide increased dramatically in slope motion process. These indices dropped sharply after entering a horizontal plane. Erosion plays an important role in landslide movement, which make slope sliding in a high speed and lead to more destructive hazards. [Conclusion] The risk degree of group geo-hazards remains severe in Tianshui City. Precipitation is the main factor that cause and aggravate the geological disasters.