Focusing on heterogeneous linear Multi-Agent System (MAS) with unknown parameters, a fixed output average consensus protocol was proposed in undirected or balanced directed network to make the output of each agent reach the average of their initial output. Firstly, each agent in the network was modeled as an unknown linear system with different order and correlation of 1 or 2, which state was updated according to the output of its own and neighboring nodes. Then, based on the model reference control method, the corresponding models were defined for the agents with different correlations. Finally, a consesus protocol was proposed to converge the output of each agent to the output of its reference model, achieving the average consesus of fixed output. The simulation with an illustrative example demonstrates the effectiveness and convergence of the proposed protocol.