A two-phase flashing flow model is developed to predict the distributions of pressure, temperature, velocity and evaporation rate in a transfer line, which is a typical example of a two-phase flow pipe in the petrochemical industry. The model is proposed based on the pressure drop model and the multi-stage flash model. The results indicate that pressure drop, temperature drop, and change of evaporation rate mainly occur. in the transition section and the function site of the transfer line. The predictions of the model have been tested with reliable field data and the good agreement obtained may lead to a better understanding of the two-phase flashing flow phenomenon, as well as demonstrating the feasibility of applying the model into the design and optimization of pipelines.