This paper is dedicated to the solution of rotor system with common fault shallow crack based on dynamical Saint-Venant’s principle and gives the interpretation of the fact that shallow crack is not detectable via the signal processing method. Analysis and proof are implemented on two prerequisites of dynamical SaintVenant’s principle: dynamical resultant of external load equal to zero and the influence of stress wave produced by external load to distal end equal to zero via breath function and elastodynamics respectively. The criterion for  whether rotor system with shallow crack yields to dynamical SaintVenant’s principle is also proposed. It concludes that most practical rotor systems with shallow crack yield to dynamical Saint-Venant’s principle and it is difficult to detect characteristics of the crack by signal processing method. Other means of shallow crack prognosis should be adopted for regular testing or online monitoring of the rotor system.