The undisturbed soil columns were collected from different soil texture sample plots in Gongga Mountain primeval forest. The infiltration and outflow process and the stable isotope (δ¹⁸O) variation characteristics were monitored and analyzed during two rainstorms in summer of 2020. The results showed that: (1) The average total runoff (4.14 and 14.61 L, respectively) of gravel rich silty loam plot A at low altitude (2 649 m above sea level) after rainfall on August 6th and 11th was larger than that of silty loam sample plot B (3 000 m above sea level) (1.45 and 11.99 L, respectively). The outflow process of plot A was also more sensitive to rainfall. (2) The variation amplitude of isotope in outflow water was smaller than that in rainfall, and the new and old water were strongly mixed in the process of infiltration. (3) There were obvious differences in the variation patterns of the source and migration path of the outflow water between the two plots. Plot A showed extremely high rainfall contribution and fast triggered preferential flow path at the initial rainfall peak, and gradually showed the characteristics of plug flow when the outflow faded. Plot B was dominated by plug flow movement in the relatively slow infiltration process, and the exchange degree of new and old water was higher. Therefore, higher organic matter, gravel content and higher rainfall intensity were beneficial to the generation of preferential flow, and the generation and transformation process of preferential flow and plug flow had a rapid response to the change of rainfall intensity, which might have a further impact on aquifer recharge and solute transport in the basin.