Abstract: Based on a mathematical model of supersonic two-phase flow in a novel cold plasma-liquid reactor, simulations were made to the momentum and heat transfer between supersonic gas stream and spray droplets during the relaxation period of the two-phase flow. The system was formed in a Layal jet by introducing spray into the supersonic plasma stream at a place below the critical section. Fast mixing and high rate volumetric transportation were the key factor for the highly dispersed supersonic two-phase flow system to match the short life span of the activated medium in cold plasma, generally in the order of magnitude around 10-2ms. Simulation results showed that tremendous potential of momentum, heat and mass transfer exist in the system especially during a short period around 0.015-0.030ms, named relaxation time τv when the droplets are accelerating after introduced into the supersonic gas stream. More detailed calculations indicated that, under a set of suitable conditions (see Table 2-3), the majority of transport process (75% of momentum and 45% of heat) can be completed within the first quarter of relaxation time. This is just in accordance to the life span of active particles in cold plasma. Furthermorg, the simulation results suggested that the large ratio of flow rate of liquid to gas as well as the small size of droplets are beneficial to the transport process in the manner of reducing τv up to 50%.