The propagation of ultrasound modulation scattering signal in two-layered and three-layered biological tissue was studied with Monte Carlo simulation. The relations with the modulation scattered light intensity and its modulation depth contributed by the thickness,absorption coefficient,scattering coefficient and anisotropy coefficient of homogeneous and layered tissue were figured out. The results of Monte Carlo simulation show that the modulation scattering light is dependent on the optical properties and configuration of tissue,but the modulated depth of the modulated light doesn′t change in the propagation and is only dependent on the optical and ultrasonic properties of biological tissue within the ultrasound zoom. So the modulated depth is a key factor in ultrasound-modulated optical tomography. These works will conduce to reconstruct the tissue imaging effectively and reliably.