In this work,NMR spectral characteristics of doxepin compounds were investigated with various experimental conditions,taking 11-（3-dimethylaminopropylidene）-6,11-dihydro-dibenzo ［b,e］oxepine-2- carbaldehyde（DP） as the testing sample.The conformation exchange of DP was confirmed.It was proved that the NMR spectral characteristics would depend on the experimental conditions,and the temperature would greatly affect the characteristics of the signals.Under the room temperature,the phenomena of signal broadening and missing were observed.As a result,the NMR signal assignment could not be accomplished.These phenomena could be improved with increase of temperature.The abnormal phenomena were effectively inhibited at 80 ℃.Finally,the complete assignment was presented and configuration was confirmed.