Horizontal linear complementarity problem has wide applications, such as in mechanical and electrical engineering, structural mechanics, piecewise linear system, telecommunication systems and so on. In this paper, we focus on the convergence conditions of the modulus-based matrix splitting iteration method proposed recently for solving horizontal linear complementarity problems. By the proposed theorems, the assumptions on the matrix splitting and the system matrices are weakened, and the convergence domain is enlarged. Numerical examples are presented to show the improvement.