Method of performing traffic steering in a wireless network system and related wireless network system

A method of performing traffic steering in a wireless network system having a base station of a first network and a user equipment camping on the first network, comprising:

• the base station setting an indicator to a first value when determining that the first network is not loaded balancedly or setting the indicator to a second value when determining that the first network is loaded balancedly;
• the base station sending the indicator, an update amount, a timer length, and at least a first radio access network (RAN) rule including a first threshold to the user equipment, wherein the update amount has a constant value larger than zero and a value of the timer length is larger than zero;
• the user equipment starting a timer when receiving the first RAN rule with the indicator set to the first value from the base station;
• the user equipment adjusting the first threshold by the update amount when the timer expires according to the timer length;
• and
• the user equipment steering traffic from the first network to a second network or from the second network to the first network when the first RAN rule is satisfied according to the adjusted first threshold.