A shell-and-tube absorber and an ammonia absorption refrigeration (AAR) system were constructed to study falling film absorption on a horizontal tube. An empirical and numerical combined method (ENCM) was developed to estimate the actual falling film thickness using parametric analysis with a simplified mathematical falling film model and experimental results. Numerical and experimental studies showed that the simplified mathematical falling film model has an exact physical response about ammonia absorption process in the absorber. The temperature difference of solution decreases with the increasing inlet coolant temperature. The heat transfer performance of a real type shell-and-tube absorber with the AAR system is a more sensitive function of the coolant inlet temperature than the solution inlet temperature. The ENCM developed in the present study can be used to estimate the actual thickness of a falling film along a horizontal tube in the shell-and-tube absorber of the AAR system, which is very difficult to measure experimentally. The ammonia absorption rate decreases with inlet temperature of a weak ammonia solution and inlet coolant temperature.