We find that cold gas can be magnetically launched from the surface of a disc with the help of the radiation pressure, if the angular velocity of the radiation-pressure-dominated accretion disc is greater than a critical value, which decreases when increasing the disc thickness Hd/R (radiation pressure). This indicates that the force exerted by the radiation from the disc does help to launch the outflow. The rotational velocity of the gas in the disc depends on the strength of the magnetic field threading the disc and the inclination κ0 (κ0 = Bz/Br) of the field line at the disc's surface. The launching condition for cold gas at the disc's surface sets an upper limit on the magnetic field strength, which is a function of the field-line inclination κ0 and the disc thickness Hd/R. This implies that a more strict constraint on the maximal jet power can be extracted from a radiation-pressure-dominated accretion disc than that derived conventionally on the equipartition assumption.