The initial dynamics of photogenerated carriers in a-Si:H/a-SiNx:H multilayers has been investigated with time-resolved laser spectroscopy. The mechanisms of thermalization and recombination of these carriers are analysed. The results also reveal that the decay time cutoff, the mobility edge and the band-tail width vary non-monotonously with respect to nitrogen content with a turning point near x = 0.85, which may result from the changes of the internal electric field and the structure of multilayer with nitrogen content.