Magnetic fields and electromagnetic forces have long been used to control the flow of a solidifying melt. In this study, low-frequency modulated electromagnetic stirring was provided by a helical permanent magnet stirrer, which can be considered as superposition of traveling and rotating magnetic fields. Flow behaviors of liquid metal driven by this helical magnetic field was investigated. Moreover, the rotating direction of the helical magnetic field was periodically reversed to form a modulated helical magnetic field. The helical magnetic field was constructed on some units of the permanent magnets magnetized in their radius directions. The liquid metal was driven by the Lorentz force via this rotating magnetic stirrer. The azimuthal and axial velocity distribution of liquid GaInSn alloy was quantitatively measured using an ultrasonic Doppler velocimetry (UDV), which revealed the time-dependent flow structure and flow pattern varying with the modulation parameters: the rotating speed of the magnetic stirrer and the modulation frequency. The main results of the velocity measurement of the liquid metal were as followed: the  azimuthal velocity of the screw flow was periodically reversed with the same modulated frequency, and the flow intensity gradually saturated when the modulation period T_m≥40 s; The axial flow resulted from the competition of large vertical vortex driven by the traveling component of the helical magnetic field and the secondary flow driven by the rotating component of the helical magnetic field. There was a critical modulated period T_m^* with respect to the reversed characteristic and flow intensity. When T_m<T_m^*, the axial velocity exhibited typically reversed flow, when T_m > T_m^*, the secondary flow appeared and gradually dominated in the axial flow pattern. The prospective goals of the present study is to develop proper magnetic fields, which can generate a three-dimensional modulated metal flow in front of solidifying front to adapt the varying casting slabs. The role of electromagnetically driven flow is to transport the solute rejected by the solidifying interface at significant distances in the melt, and to periodically reverse its circulation so that macrosegregation is minimized.