An electrical power assisted steering device has a lower shaft, an upper shaft module, a pre-torsion module and a torsion bar. The upper shaft module has an upper shaft, a detecting unit mounted around the upper shaft, and an assembling space annularly formed between the detecting unit and the pushing portions. The pre-torsion module is mounted in the assembling space and has a maintaining unit mounted around the upper shaft and in the assembling space, multiple sliding blocks mounted in the maintaining unit and respectively abutting the upper shaft, and multiple springs mounted in the maintaining unit and respectively abutting between the maintaining unit and the sliding blocks. The torsion bar is connected between the upper shaft and the lower shaft.