The controlling of molecular orientation and structural ordering of organic semiconductors is crucial to achieve high performance electronic devices. In this work, large-area highly oriented and ordered films of an excellent electron transporter Poly{[N,N-bis(2-octyldodecyl)-1,4,5,8-naphthalenedicarboximide)-2,6-diyl]-alt-5,5-(2,2-bithiophene)} (P(NDI2OD-T2)) are achieved by improved solution-cast in high magnetic field. Microstructural characterizations reveal that the chain backbones of P(NDI2OD-T2) are highly aligned along the applied magnetic field in the films. Based on the synchrotron-based X-ray diffraction analysis of the polymer films cast from different solvents, a mechanism which controls the alignment process is proposed, which emphasizes that molecular aggregates of P(NDI2OD-T2) preformed in the solution initiate magnetic alignment and finally determine the degree of film texture. Furthermore, the time-modulated magnetic field technique is utilized to effectively control the orientation of -conjugated plane of the backbones, thus the degree of face-on molecular packing of P(NDI2OD-T2) is enhanced significantly. Thin film transistors based on the magnetic-aligned P(NDI2OD-T2) films exhibit an enhancement of electron mobility by a factor of four compared to the unaligned devices, as well as a large mobility anisotropy of seven.