With Multi-channel MAC protocols, transmission pairs can send data packet simultaneously. Hence, higher network throughput can be achieved. The developing of 802.11s standard provides the power saving model multi-channel MAC protocol (PSM-MMAC) framework, which divides into negotiation phase and data phase. All the users select appropriate channels to transmit data in dedicated channel during negotiation phase, then the users will switch the transceivers to the selected channel to transfer data packet by RTS/CTS handshake. While taking the size of data window constraints, therefore how to design the data packet size is the key issue of PSM-MMAC in saturation network case. In this paper, we propose a new policy that can adjust the size of data packet based on the channel occupancy and the fairness of user access for unsolicited Grand Service, which has strict requirements on the delay jitter and maximum reserved traffic rate. We also develop the probabilistic statistical analysis model to compare the performance of previous policy and our proposed policy. Simulation results verify our analysis model, and show our policy has the superior performance of the PSM-MMAC to the previous scheme.