Numerical studies on a spatially developing supersonic mixing layer were performed by means of large-eddy simulation. Focusing on the effect of transverse forcing on the passive scalar mixing characteristic of mixing layer. The results for the baseline case without external forcing are in agreement with the analytical evidence. Afterwards, the effect of transverse forcing on the scalar structure, scalar thickness and volumetric entrainment ratio of mixing layer were analyzed. Results indicate that the frequency and amplitude of transverse forcing alter the growth rate and entrainment ratio of the passive scalar mixing significantly. High-frequency forcing magnifies the scalar thickness and volumetric entrainment ratio in the near field of mixing layer. However, the growth rate in the far field is increased by the low-frequency forcing. The entrainment ratio is, to be a large extent, dominated by the large-scale eddy entrainment process. The multiple-frequency forcing seems to validly enhance the scalar mixing in supersonic mixing layer.