A cinnamoyl coenzyme A reductase (CCR, EC 1.2.1.44), one of the key enzyme involved in lignin biosynthesis, was cloned from Populus tomentosa (Chinese white poplar). At the same time, a 4CL1 gene was cloned from P. tomentosa, too. The two genes were subcloned in pQE31 vector and expressed in Escherichia coli M15. Both of them were purified by Ni-NTA. Purified CCR protein was digested by trypsin and analyzed by HPLC-MS; the peptide segments had 27% similarity with the sequence of the CCR protein. 4CL was thought to be a neighbor enzyme of CCR in lignin biosynthesis. In this paper, a 4CL1 from P. tomentosa was cloned, and its enzyme reaction products were extracted for the substrates of CCR. Three 4CL1 enzyme reaction products were monitored by HPLC-MS and then the CCR enzyme reaction was detected by GC-MS. In the CCR reaction, the three corresponding aldehyde (p-coumaraldehyde, caffealdehyde, and conifer-aldehyde) were detected and identified by Frontier3 software. The results showed that the CCR that we cloned from P. tomentosa had affinities with 4CL1 enzyme reaction products and a ptCCR that was cloned from aspen (Li et al., Plant Cell Physiol 46(7):1073-1082, 2005) only had affinity with feruloyl-CoA. The different results maybe depend on the different study method. The method of exacting 4CL enzyme products as the substrates of CCR in the paper was reliable and can be used in lignin biosynthesis network to detect the enzymes in the neighborhood that depended on the polarity of the substrates and products. This CCR gene had eight homology sequence CCR gene when a BLAST was conducted in Populus trichocarpa genome database. The CCR homology genes in Populus suggested that some CCRs may take part in the lignin biosynthesis, too. The gene family would be the hot spot in the future study.