Laccase, widely distributed in fungi lacking high substrate specificity, plays an important role in lignin degradation in nature and environmental protection. In order to determine or estimate the laccase production during the fermentation of liquid media, the authors studied the full-length wave scan on the rough fermentation liquid of the Pleurotus ostreatus, which produces laccase high. Combined with the normal chemical method and diameter of the laccase and mycelium stain, which grew on the PDA (potato dextrose agar) plate with guaiacol added, we could get the exact information of laccase production. The result showed that the laccase activity increased in a rapid way in the first 5 days during the fermentation process, remained almost at the same level in the following 4 days, then increased rapidly until the 11 day, which was 148.7 U·L-1, increased 17.9 times. The diameter of laccase and mycelium stain increased with the culture time. The number of the wave peaks around 300 nm had a positive correlation with the laccase production; the peak width of OD over 1.5 around 300 nm had a positive correlation with the laccase production, which ranges from 5 nm on the first day to 80 nm on the 11th day. The light absorption line between the wavelengths 300 and 400 nm had a positive correlation with the laccase production with peaks at 349, 365 and 388 nm, and at 365 the peak gets its highest. Using these parameters, the authors could get the general production of the laccase production of liquid fermentation. Compared with the normal chemical method, the full-length wave scan method is much easier, cheap and simple. Furthermore, there are no special chemical substances used. It is really a new method for the evaluation and determination of laccase.