A combined pancreatic β-cells model was given by two important slow negative feedback mechanisms. Diverse patterns of electrical bursting are robust to fast, medium and slow oscillation periods. Therefore, bifurcation mechanisms and topology types of these bursting patterns can be surveyed by fast/slow analysis of only fast bursting, when the faster slow variable Css is regarded as a bifurcation parameter, the faster slow variable Cer has no effect on the fast subsystem and the slower slow variable a is nearly constant.