A method for controlling a shift-by-wire (SBW) transmission may include detecting whether a shut-down condition is met in a state in which a parking gear stage of a vehicle is released, when it is detected that the shut-down condition is met, starting to drive a parking release device of maintaining release of a parking gear on the basis of an oil pressure of a parking piston or an RPM of an engine, and controlling an ON/OFF duty ratio of electric power of driving the parking release device on the basis of oil pressure of the parking piston, after starting to drive the parking release device.