The quantitative characters of litter layer were investigated in the south side of Liupan Mountains of Northwest China for a larch (Larix principis-rupprechtii) and birch (Betula albo-sinensis) stand. The thickness of litter layer of the larch and birch forest was 3.6 cm and 4.0 cm, and their corresponding weight was 16.83 t/hm2 and 9.04 t/hm2 respectively. The water-holding capacity of unit weight of litter (1.0 kg/m2) measured by a soaking method was 1.8 mm for larch and 2.7 mm for birch. The rainfall interception process was studied under simulated rainfall within the normal rainfall intensity range of this region (< 30 mm/h). It showed that the interception capacity of litter increased with rainfall intensity. The maximum depth of interception of unit weight of litter (1.0 kg/m2) under the rainfall intensity of 30 mm/h was 1.5 mm for larch and 2.6 mm for birch, accounting for only 83.3% and 96.2% of the water-holding capacity determined by soaking method. However, the water soak-up processes in the two different experiments were essentially consistent with each other. The water absorption rate of litter was influenced by its moisture condition. A mechanism based model for describing the rainfall interception process of litter of the two tree species was developed, and the model parameters were fitted based on the measured data both from the soaking experiment and simulated rainfall interception experiments.