The multistep collocation method is introduced to solve integral-algebraic equations of index 1. The existence and uniqueness of the multistep collocation solution are proved. The convergence of the perturbed multistep collocation method is also investigated, which extends and includes the analysis of the multistep collocation method without perturbed terms. Some numerical experiments are given to illustrate the theoretical results. Keywords Integral-algebraic equations • Index 1 • Multistep collocation method • Existence and uniqueness • Convergence Mathematics Subject Classification 65R20 1 Introduction Volterra integral equations (VIEs) have rich applications in physics, biology, chemistry, etc. Generally, VIEs are divided into two classes: first-kind VIEs and second-kind VIEs. For first-kind VIEs, the unknown function only appears inside the integral, but for second-kind Communicated by Antonio José Silva Neto.