Apparatus and method for controlling steering system of vehicle

An apparatus and a method for controlling a vehicle steering system, which are capable of improving a sense of unity between driver's driving input and vehicle motion and the stability of the vehicle by estimating a side slip angle and performing return control, may include an assist torque determination module for determining assist torque for steering assist, a return control module for determining steering return torque for returning the steering wheel to a neutral position, a side slip angle estimation module for estimating a side slip angle, a side slip control module for determining a return control gain value in a response to the estimated side slip angle, a correction unit of correcting the determined steering return torque in a response to the return control gain value, and a torque compensation unit of determining final assist torque by compensating the determined assist torque with the corrected steering return torque.