Firstly, a mathematical model of asymmetrical bi-directional multi-relay system is presented. Using opportunistic relay and network coding, an outage probability expression of the mode is given,which is proved accurate by simulation. Secondly, we analysis the outage performance of the model at different relay node location, different number of relay node, and different power allocation coefficient, indicating the inherent relationship between power allocation coefficient and relay node location, the number of relay node, and the total power of the system. Simulation results indicate that optimal system outage performance of asymmetrical bi-directional multi-relay system is attained when power allocation coefficient is chosen at 0.6 or about 0.6 while using Network Coding and opportunistic relay, and indicating that the location of relays is important while investigating the problem of outage probability.