The performance of NH3-H2O inner vertical adiabatic falling film absorption was studied based on the experimental set-up. The potential of absorption was just given by the inlet sub-cooling of the weak solution. Variable working conditions were performed and tested. A novel evaluation method related to Sherwood number was derived from the conservation and transfer equations, which was used to evaluate the experimental data. The experimental results show that transfer rates and Sherwood numbers increase with the growing of Reynolds numbers. The mass transfer process is enhanced by increasing absorption pressure or inlet sub-cooling. The mass transfer performance of coupled heat and mass transfer combined with inlet sub-cooling is higher than inlet sub-cooling or coupled heat and mass transfer only. Sherwood correlation has been fitted based on the experimental data to evaluate the adiabatic falling film absorption. It could be used in designing falling film absorber whose heat and mass transfer are independent of each other.