Based on non-destructive detection, the fluorescence spectrum of living leaves of poplar was collected by laser as excitation light source. Sensitive fluorescence spectrum band of water use efficiency was arrived through analyzing spectrum data. A model depending on the modification of leaves temperature was established. The functional relation between water use efficiency and fluorescence spectrum was described and the multiple correlation coefficients R2 were 0.975. The correlation coefficient r between calculated values and measured values was 0.8746 by verification test.