The experimental data of interspike intervals were recorded in a neural pacemaker,and the presence of period-adding bifurcation cascade was demonstrated.By detecting unstable periodic orbits from the chaotic interspike intervals located in between period-n bursting and period-(n+1) bursting,it is found that the unstable period-n orbit can be detected from the chaotic interspike intervals near the period-n bursting,and the unstable period-(n+1) orbit exists in the chaotic interspike intervals near period-(n+1) bursting.Moreover,the unstable period-n orbit also can be detected.For such a phenomenon the theoretical analyses are presented from the point of view of the nonlinear dynamics by way of the pancreatic β-cell model suggested by Sherman.The direct causes of emergence of the phenomenon are shown to be the intermittency of type Ⅰ and that of type Ⅲ,and they are induced by the saddle-node bifurcation and period-doubling bifurcation,respectively.Consequently,the mechanism for identification of chaotic firing interval is revealed.At the same time,the universality of the phenomenon is elucidated to a certain extent.Furthermore,a novel method is put forward for identifying the chaotic firing interval.