In this study, an analytical expression is derived based on the extended Huygens-Fresnel principle for the spectrum of a Gaussian Schell-model (GSM) beam that propagates in biological tissues. The spectral change of the GSM beam during propagation is studied based on the normalized spectrum and the relative spectral shift. Results show that the spectral blue shift, red shift, and rapid transition can be observed when the GSM beam propagates in biological tissues, and they are dependent on the off-axis distance, propagation distance, type of biological tissue specimen (specifically the refractive-index structure constant of tissue turbulence), and spatial correlation length. As the propagation distance increases, the refractive-index structure constant increases, meaning that the turbulence of biological tissue becomes stronger. Meanwhile, as the spatial correlation length increases, the position where spectral rapid transition occurs is farther and the transition qualities correspondingly decrease; furthermore, the spectral rapid transition becomes increasingly weak and the propagation position where a transition can be observed from the spectral red shift to blue shift becomes increasingly distant. With increasing values of the refractive-index structure constant and the spatial correlation length, the off-axis distance associated with the spectral rapid transition will also increase, i.e., the distance between the observation position and the propagation axis will increase.