A method for post-authenticating user equipment (UE), a controller and a network system are disclosed. The method includes: determining whether an external channel quality of at least one gateway is higher than a threshold when UE connects to an access point (AP) routed by the gateway; when the external channel quality is not higher than the threshold, authenticating the UE incompletely and controlling the AP to limit a network capability providing to the UE; estimating a future channel capacity of the gateway; calculating a priority weight of each UE; selecting candidate UE from the UEs according to the future channel capacity and the priority weight of each UE; arranging an authenticating mechanism for completely authenticating the candidate UE at a timing point corresponding to the future channel capacity.