The spectral dependence of SHG intensity in biological tissues is an optical property that is not fully understood so far. In this paper, we will investigate this problem in detail through experiments. Through examining different biology tissues, it is found that SHG intensity drops down from shorter to longer wavelength from 375 nm to 460 nm. By comparing these curves with 1/n dependence, the n is found to vary from 4.5 to 8.5. These patterns can not be fully explained by scattering properties of collagen. Other factors such as direct generation of SHG may have contribution to these wavelength dependence patterns, which needs further investigation.