According to the three-dimension unsteady suspended sediment transport equation achieved by turbulent diffusion theory, and by using similarity theory, the similar conditions of suspended sediment transport are discussed preliminarily. The result indicates that the similar condition of settlement is the same as that of suspension. Many investigators have derived different suspension similar conditions which are incompatible with above mentioned condition, of which the key reason is that, in resolving the sediment turbulent diffusion scale, three incompatible conditions in distorted model have been applied at the same time, i.e. the friction velocity, the logarithmic velocity distribution and the Karman constant scale.