Abstract: The hydrodynamic calculation method in use is the "hydrodynamic calculation method for hot water boiler".The disadvantage of this method is that it is hard to ascertain the working medium flux of each tube and furthermore,the working point is inaccurate.To overcome the inaccurateness,a numerical hydrodynamic calculation method called hydrodynamic loop analysis method or loop analysis method was investigated.It took those factors affecting the flux of each tube into account and obtained the accurate flux of the working medium in each tube under the condition that the heat load,the parameter of structure and the flow resistance coefficient of each tube were supplied.The hydrodynamic characteristic of a single loop was researched by both the general method and the loop analysis method under the same parameters to demonstrate the correctness of the hydrodynamic loop analysis method.Then the hydrodynamic characteristic of a hot water boiler with natural circulation was calculated and analyzed by the hydrodynamic loop analysis method as well as the general one.Results show that hydrodynamic loop analysis method is more accurate and acceptable.