Microtremors observing is a kind of convenient and effective test method for site soils dynamic characteristics. According to the character of Rayleigh wave is the main energy in microtremors of sites and based on the Rayleigh wave dispersion curve of layered soils, the relationship between Rayleigh wave length and theoretical calculation depth corresponding to microtremors predominant frequency is analyzed. Computation results show that, the difference characteristics of soil layers shear wave velocity of site are the main influencing factor. If the difference of shear wave velocities is small or a certain wave velocity is dominant, then the ratio of theoretical calculation depth to Rayleigh wave length is about 1/4a ( a is the ratio of Rayleigh wave length to shear wave length). That is, is close to the result of uniform site. Conversely, the bigger difference of soil layers wave velocities, the further away to the ratio of 1/4a, and 1/4a is the upper limit.