Hydraulic stability is an important criterion to evaluate hydronic networks .In this paper, quantitative definition of hydraulic stability is presented and the calculation method is introduced in detail .To a network, this criterion can be calculated according to the structure and the hydraulic condition of it , and it can also be calculated according to the measured data online . Then, how helpful the value of the criterion in understanding the performance of a network is discussed with the simulation of the control process .At last, several typical networks are discussed as　 case studies: direct return network, reverse return network and distributed -pumps system.