Recently, homomorphic message authentication code (MAC) schemes have been proposed to resist against pollution attacks in network coding. However, existing methods face a common challenge: the generated MAC $(t)$ belongs to a small finite field $({\hbox{\rlap{I}\kern 2.0pt{\hbox{F}}}}_{q})$, which means that an adversary could attack by randomly guessing the value of $(t)$, and succeed with probability $(1/q)$. Since $(q)$ is a predetermined system parameter which is typically set as $(2^8)$, the resulted security $(1/256)$ could be unsatisfactory in practice. In this paper, we propose an efficient homomorphic MAC for authentication in network coding. The proposed method achieves a reliable security parameter $(1/q^l)$ using only one key, where $(l)$ could be chosen according to different security requirements. Compared with previous approaches that using multiple tags, the proposed homomorphic MAC has both low computation and communication overheads.