LoRa network uses wideband linear frequency modulation techniques and employs different spreading factor (SF) settings to achieve different transmission rates. When there are many nodes in the network and collision occurs, nodes with higher spreading factor have lower transmission speed and longer transmission time, and are more prone to collision. The transmission success rate is also relatively low. Therefore, when each spreading factor in the network allocates the same number of nodes, it is not fair for low-rate transmission, because its channel takes longer and collision probability is greater. This paper proposes an allocation mechanism based on the spreading factor (FSA). The high spreading factor is more prone to collision and its transmission success rate is also low. Therefore, fewer nodes are assigned to the high spreading factor and low spreading factor allocates more nodes, so that the transmission success rate of different spreading factors is the same. Consequently the problem of fairness caused by the unreasonable allocation of the spreading factor of the nodes in the collision network is effectively solved, the probability of retransmission of high spreading factor nodes is reduced, and the stability of the network is greatly improved.