Lignin genetic engineering can effectively reduce the lignin content ofthe cell walls in poplars, while improving utilization characteristic of poplars as the lignocellulosic material. In this paper, the transgenic poplars with C3H downregulated and control poplars were selected. Fourier transform infrared (FTIR) spectroscopic was used to quickly characterize the lignin content and the other cell wall constituents of transgenic poplars with C3H-downregulated. CLSM and histochemistry reaction were used to reveal the influence of lignin content reduction on their micro-area distribution in situ. The results showed that the FTIR spectrum shape, the number and wavenumbers of characteristic bands of transgenic poplars and control poplars were similar which means that the reduction of C3H activity had no effect on the chemical composition of poplar cell walls. However, the height ratios of I1 508/I1 379, I1 508/I1 425, and I1 508/I1 740 associated with lignin content reduced 8.2% to 9.5% in transgenic poplars compared with control poplars. The difference in intensity of spectrum peaks between the transgenic poplars and the control poplars illustrated that the C3H-downregulated can change the cell wall constituents content of the transgenic poplars. The CLSM images showed that the micro-area distribution trends of the lignin of the cell walls in transgenic poplars and control poplars were similar, and the sequence of lignin distribution was the cell wall corner>the middle lamellar>the second cell wall, and the images also showed the lower lignin content of the cell walls in transgenic poplars than that of the control group. The Wiesner and Mule staining images also showed that S lignin monomer was uniformly distributed in fiber cell walls of the two kinds of poplars, and the G lignin monomer deposition was nonhomogeneous, the sequence was also the cell wall corner>the middle lamellar>the second cell wall. These results also illustrated that reducing the activity of C3H gene had no influence on distribution trend of G and S lignin monomer, but changed the lignin monomer content of the cell walls of fiber and vessel in transgenic poplars.